Abstract:
In a spring piston airgun, a compression piston is provided for longitudinal translation within a compression tube in response to a motive force. The compression piston includes a main piston body and a piston head, wherein the piston body and piston head are coupled for partial independent translation along a longitudinal axis. A resilient compressible bushing is longitudinally intermediate a portion of the piston body and the piston head, such that upon a deceleration of the piston head, the piston body is not immediately acted upon by the deceleration, rather the bushing absorbs a portion of the deceleration and radially expands to contact the compression tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Applicant claims the benefit of previously filed provisional patent application 61/822,177 filed May 10, 2013, the disclosure of which is hereby expressly incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present disclosure is directed to airguns and particularly to spring piston airguns and more particularly to a compression piston for a spring piston airgun. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    In one configuration, the present disclosure is directed to an apparatus having a barrel, a compression tube having transfer port fluidly connected to the barrel, a compression piston at partially disposed within the compression tube and moveable within the compression tube between a first position and a second position, the compression piston having piston body and a piston head, a seal connected to the piston head and forming a sealed interface with an inside surface of the compression tube and a radially expandable bushing connected to the piston body, the bushing radially expanding in response to a longitudinal, such as a longitudinally compressive, force on the bushing, the radial expansion sufficient to contact the bushing with an inside surface of the compression tube and decelerate the piston body, such as relative to the compression tube. 
         [0004]    In a further configuration, the compression piston includes a plurality of tail guides extending radially from the compression piston, the plurality of tail guides contacting the inside surface of the compression tube. It is understood the plurality of tail guides locate a portion of the compression piston relative to an inner surface of a compression tube. 
         [0005]    A spring can be connected to the compression piston to move the compression piston from a first position in the compression tube to a second position in the compression tube. The spring can be a metal coil spring, a pneumatic or a gas spring. 
         [0006]    Alternatively, the apparatus includes a compression piston having a main piston body, a piston head moveably connected to the piston body to be longitudinally displaceable relative to the piston body, the piston head including a seal and a radially expandable bushing contacting the piston head and the main piston body, the bushing radially expanding in response to relative longitudinal movement of the piston body towards the piston head. 
         [0007]    The compression piston can be sized to be slideably received within a compression tube. It is further contemplated that a plurality of tail guides can radially project from the compression piston to locate the compression piston concentric with the compression tube. Specifically, the plurality of radially projecting tail guides locate a portion of the compression piston relative to the inner surface of a compression tube. The compression piston can also include a seal selected to provide a sealing interface with the compression tube. 
         [0008]    A method is disclosed which includes using a spring to urge a compression piston to move within a compression tube towards a barrel end of the compression tube, the compression piston having a piston head and a piston body, the piston head being longitudinally displaceable relative to the piston body; and radially expanding a bushing intermediate the piston head and the piston body a sufficient radius to decelerate at least a portion of the compression piston relative to the compression tube. 
         [0009]    It is understood the spring can be a coil spring or a gas spring. In the gas spring configuration, the gas spring includes a gas spring body defining a sealed interior chamber containing a compressed gas and a gas spring piston extending into and moveable relative to the sealed interior chamber, the interior chamber retaining the compressed gas when the gas spring piston moves. 
         [0010]    A further method is provided of mounting a spring in a spring piston airgun to urge a compression piston to move within a compression tube towards a barrel end of the compression tube, the compression piston having a piston head and a piston body, the piston head being longitudinally displaceable relative to the piston body; and locating a radially expandable bushing intermediate the piston head and the piston body, the radially expandable bushing expanding in response to longitudinal displacement of the piston head relative to the piston body a sufficient radius to decelerate at least a portion of the compression piston relative to the compression tube. 
         [0011]    The method can include using a coil spring or a gas spring as the spring. The gas spring can include a gas spring body defining a sealed interior chamber containing a compressed gas and a gas spring piston extending into and moveable relative to the sealed interior chamber, the interior chamber retaining the compressed gas when the gas spring piston moves. 
         [0012]    Alternatively, an apparatus is provided having a barrel, a compression tube having transfer port fluidly connected to the barrel, a compression piston at partially disposed within the compression tube and moveable between a first position and a second position, a spring contacting the compression piston to selectively move the compression piston between the first position and the second position and a plurality of tail guides extending radially from the compression piston, the plurality of tail guides contacting an inside surface of the compression tube. 
         [0013]    The tail guides can be formed of a different material than the compression tube and the compression piston. The tail guides can be located at a variety of circumferential locations on the compression piston. 
         [0014]    Thus, a compression piston is provided for an airgun having a compression tube slideably receiving the compression piston and a spring selectively moving the compression piston relative to the compression tube, the compression piston comprising: a plurality of radially projecting tail guide locating a portion of the compression piston relative to an inner surface of the compression tube. It is understood, the gas spring can contact the compression piston. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    The drawings provided in the present disclosure are provided solely to better illustrate particular embodiments of the present invention, and specifically do not provide an exhaustive or limiting set of embodiments of the present invention. 
           [0016]      FIG. 1  is a partial side elevational view in cross section showing an airgun with a configuration of the present compression piston in a fired position. 
           [0017]      FIG. 2  is a partial side elevational view in cross section showing an airgun with a configuration of the present compression piston in a cocked position. 
           [0018]      FIG. 3  is an exploded perspective view of one configuration of the present compression piston. 
           [0019]      FIG. 4  is a side view cross section of the exploded compression piston of  FIG. 3 . 
           [0020]      FIG. 5  is a side view of the assembled compression piston of  FIG. 3 . 
           [0021]      FIG. 6  is a side view cross section of the compression piston of  FIG. 5 . 
           [0022]      FIG. 7  is an enlarged side view cross section of the head of the compression piston of  FIG. 5 . 
           [0023]      FIG. 8  is an exploded perspective view of a second configuration of the present compression piston. 
           [0024]      FIG. 9  is a side view cross section of the exploded compression piston of  FIG. 8 . 
           [0025]      FIG. 10  is a side view of the assembled compression piston of  FIG. 8 . 
           [0026]      FIG. 11  is a side view cross section of the compression piston of 
           [0027]      FIG. 10 . 
           [0028]      FIG. 12  is an enlarged side view cross section of the head of the compression piston of  FIG. 11 . 
           [0029]      FIG. 13  is a schematic representation of the compression piston relative to the compression tube, with the spring omitted, during an early portion of the firing cycle. 
           [0030]      FIG. 14  is a schematic representation of the compression piston relative to the compression tube, with the spring omitted, during an intermediate portion of the firing cycle. 
           [0031]      FIG. 15  is a schematic representation of the compression piston relative to the compression tube, with the spring omitted, at the terminal portion of the firing cycle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The present system can be used in a variety of configurations to reduce deceleration induced vibration during firing of an airgun. In one configuration, the present system is used in an airgun to selectively provide compressed air to propel a bullet or projectile, and in a more specific configuration, the system is employed in a spring piston airgun  10 . 
         [0033]    For purposes of description of the airgun configurations, the term front or forward means towards the muzzle and the terms rear or rearward mean towards the butt end (or operator). The term longitudinal or longitudinal axis is used to describe a direction along the barrel, parallel to the barrel or along the longer dimension of the respective component. 
         [0034]    Referring to  FIGS. 1 and 2 , in the configuration used in the spring piston airgun  10 , the airgun generally includes a barrel  12 , a stock  16 , a compression tube  22 , a trigger mechanism  30 , a cocking mechanism  40 , a spring  50  and a compression piston  60   
         [0035]    The barrel  12  is supported by the stock  16  and extends along a longitudinal axis from a breach to a muzzle. The breach is fluidly connected to a transfer port  24  of the compression tube  22 . The compression tube  22  is well known in the art and is typically formed of a metal for performance, safety and durability factors. The compression tube  22  includes an inner or inside wall or surface  23 . 
         [0036]    The cocking mechanism can be any of a variety of mechanisms including but not limited to cams or levers, including cocking arms and break barrel constructions. The cocking mechanism allows the user to move the spring from a fired configuration,  FIG. 1 , to a cocked configuration,  FIG. 2 . Thus, energy is input into the airgun  10  for selective conversion into motion of the projectile through the barrel  12 . 
         [0037]    The compression piston  60  is moveable within the compression tube  22  to move between a cocked position and a fired position. Movement of the compression piston  60  from the cocked position to the fired position in response to the force of the spring  50  forces pressurized air through a transfer port to the breach to propel the projectile from the breach and through the barrel  12 . 
         [0038]    The spring  50  can be any of a variety of configurations including metal coil or helical springs, composite or alloy coil or helical springs as well as gas springs or struts. Each of these types of springs is well known in the industry. In one configuration, seen in  FIGS. 1 and 2 , the spring  50  is a longitudinal spring, that can be longitudinally compressed or extended but returns to a former configuration when released. In an alternative configuration, the spring  50  is a helical metal coil which expands and contracts generally along a longitudinal axis of the spring. Referring to  FIGS. 1 and 2 , the spring  50  is a gas spring having a gas spring body  52  defining a sealed interior chamber  54  containing a compressed gas  56  and a gas spring piston  58  extending into and moveable relative to the sealed interior chamber, the interior chamber retaining the compressed gas when the gas spring piston moves. Thus, as the gas spring piston  58  is forced into the sealed interior chamber  54  during cocking, the pressure in the internal chamber rises even further as the piston reduces the effective volume of the interior chamber. The increased pressure thus creates a force on the piston  58  urging the piston from the interior chamber  54 . 
         [0039]    As seen in  FIGS. 3-12 , the compression piston  60  is a multi-piece construction having a piston head  70 , a piston body  90 , and resilient, radially expanding, bushing  110 , wherein the piston body is longitudinally displaceable relative to the piston head. For purposes of description, the piston head  70  is the portion of the compression piston  60  that is forward of the piston body  90 . That is, the piston head  70  is nearer to the muzzle (or the transfer port  24 ) than the piston body  90 . 
         [0040]    Although the piston head  70  and piston body  90  could have numerous constructions, for purposes of the present description, the piston body is a generally cylindrical elongate member having a front or leading end  96  and a rear or trailing end  92 . The piston body  90  includes an elongate channel  93  for accommodating the cocking mechanism  40 , as known in the art. 
         [0041]    The rear end  92  of the piston body  90  can be open and the front end  96  includes an aperture  97  defining a radially inward projecting shoulder  98 . 
         [0042]    The front end  96  can have any of a variety of profiles from flat faced (perpendicular to the longitudinal axis) as seen in  FIGS. 3-7 , to tapered, stepped or conical, as seen in  FIGS. 8-12 . 
         [0043]    The compression piston  60 , and in select configurations, the piston body  90  can include a plurality of tail guides  120 . The tail guides  120  create multiple points of contact with compression tube  22 , wherein these points of contact maintain the piston body  90 , and hence compression piston  60 , in a concentric orientation with the compression tube. The concentric orientation of the piston body  90  with the compression tube  22  increases efficiency of the compression piston  60  and reduces noise upon movement of the compression piston from the cocked to the fired position. Thus, the tail guides  120  can be located on the piston head  70 , the piston body  90  or both the piston head and the piston body. 
         [0044]    The multiple points of contact, in the configuration shown in  FIGS. 3-12 , can be three points. These three points are the minimum number of contacts required to keep the compression piston  60  concentric to the compression tube  22 . Although the tail guides  120  can be located at a variety of circumferential positions, it has been found advantageous to symmetrically locate the tail guides about the 360 degree circumference of the piston body  90 . Thus, the use of three tail guides  120  located at 120° intervals minimizes the frictional loses associated with tail guides, by reducing friction by keeping the rear end  92  of the piston body  90  and hence piston body isolated from contact with the compression tube  22 . It is understood the location of the tail guides  120  is not specific per se. That is, the tail guides  120  can be located anywhere on the circumference of the piston body  90  and anywhere along the longitudinal dimension of the piston body  90 . 
         [0045]    As seen in  FIGS. 3 ,  4 ,  6 ,  8 ,  9  and  11 , the tail guides  120  can be generally spherical or hemispherical and are retained within corresponding recesses  123  in the piston body  90 . However, the tail guides  120  are not limited to spheres or hemispheres, and can have faceted, apex, line or point contact surfaces with the compression tube  22 , specifically an inner or inside surface or wall of the compression tube. It is also understood the number of tail guides  120  can range from one to a multiple such as 10 or more, depending on the desired operating characteristics and design construction. 
         [0046]    Further, although the tail guides  120  are set forth as buttons, it is understood the tail guides could have any of a variety of configurations, including but not limited to arcs, ridges, helical sections, as well as lines either parallel to, inclined or perpendicular to the longitudinal axis. 
         [0047]    Thus, it is the tail guides  120  that contact the compression tube  22  (specifically the inner wall  23  of the compression tube), rather than the material of the piston body  90  contacting the compression tube. The use of the tail guides  120  rather than a ring or sleeve extending about the compression piston  60 , further reduces the frictional losses by reducing the total contact area between the compression piston (the piston body  90 ) and the compression tube  22 , while keeping the compression piston stable, and off the compression tube wall  23 . 
         [0048]    The tail guides  120  not only reduce friction during the firing cycle, but the tail guides reduce the metal to metal contact between the compression tube  22  and the compression piston  60 , thereby further reducing and damping vibration. A further benefit lies in the cocking of the compression piston  60 , as the tail guides  120  contribute to smoother movement of the compression piston relative to the compression tube  22  during cocking of the air gun. 
         [0049]    The tail guides  120  can be formed of a variety of materials, including but not limited to polymers such as nylon, PTFE and PTFE coated nylon. While numerous configurations of the tail guides  120  are non-metal, it is understood various alloys and metals, such as oil impregnated bronze can be used for the tail guides. 
         [0050]    Referring to  FIGS. 3 ,  4 ,  6 - 9 ,  11  and  12 , the piston head  70  includes a rearwardly projecting stem  72 , a radially projecting flange  76  and a seal retainer  78 . The stem  72  has a diameter sized to slidingly pass through the aperture  97  in the front end  96  of the piston body  90 . The stem  72  has an axial (longitudinal) dimension sufficient to engage the bushing  110 , as set forth below. The flange  76  is sized to preclude passage of the piston head  70  through the aperture  97 . 
         [0051]    The piston head  70  carries a piston seal  80  for forming a sliding sealed interface with the inside surface of the compression tube  22 . The piston seal  80  is well known in the art in both material and structure. Similarly, the engagement of the piston seal  80  to the piston head  70  can be provided as known in the art, such as by seal retainer  78  which is in the form of a flared or tapered surface selected to engage a corresponding surface on the seal  80 . 
         [0052]    A capture piece  84  such as a bolt (or nut) is sized to pass through the rear end  92  of the piston body  90  and engage the piston head  70 , such as by engaging the stem  72 . The capture piece  84  includes a portion having a radial dimension precluding passage through the aperture  97 . Although the capture piece  84  is shown as a bolt having external threads for engaging corresponding internal threads on the stem  72  of the piston head  70 , it is understood that any of a variety of interconnect structures can be used to retain the piston head  70  to the piston body  90 . That is, the stem  72  can include external threads with the capture piece  84  can be a nut having internal threads for engaging the stem. Alternatively, rotatable bayonet type interlocks can be used. Similarly, snap or detent connections can be employed to retain the piston head  70  relative to the piston body  90 . It has been found advantageous for the piston head  70  to be able to rotate relative to the piston body  90 . Thus, while the capture piece  84  can locate and retain the piston head  70  at a fixed rotational position with the piston body, in select configurations, the piston head can rotate relative to the piston body. 
         [0053]    Referring to  FIGS. 6 ,  7 ,  11  and  12 , the capture piece  84  can include a recess  85  for cooperatively receiving a tool such as a hex key, a screw driver or even socket driver for providing adjustment of the longitudinal spacing for the bushing  110 . That is, the longitudinal spacing between the portion of the piston head  70  and the portion of the piston body  90  receiving the bushing  110  can be set to be greater than, equal to or less than the corresponding longitudinal dimension of the bushing. Thus, by selectively setting, or preloading, a compressive force on the bushing  110 , the reaction of the bushing (amount of radial expansion) can be set for a given compression piston  60  and airgun  10  or system. 
         [0054]    As seen in  FIGS. 3-15 , the bushing  110  is captured between a portion of the piston head  70  and a portion of the piston body  90 . In one configuration, the bushing  110  is retained between the front end  96  of the piston body  90  and the flange  76  of the piston head  70 . 
         [0055]    The bushing  110  has an outer wall  112 , an inner wall  114  defining a central aperture  115  sized to receive the stem  72  of the piston head  70 , a front end  116  and a rear end  118 . The outer wall  112  can be a generally cylindrical surface. However, it is understood the outer wall  112  can be non-cylindrical and include ridges or protuberances. In addition the outer wall can be tapered such as frustoconical, wherein the largest diameter is equal to or less than the diameter of the compression tube  22 . 
         [0056]    The front end  96  of the piston body  90  and the rear end  118  of the bushing  110  can be substantially planar (perpendicular to the longitudinal axis), inclined such as wedged or tapered as well as featured such as ridges or protuberances. Thus, the bushing  110  and the piston body  90  can define engaging surfaces, wherein the engaging surfaces are non-perpendicular to the longitudinal axis. The engaging surfaces can be selected to enhance radial expansion of the bushing  110  during deceleration of the compression piston  60  during the firing cycle. 
         [0057]    The bushing  110  can be solid, hollow, webbed, non-homogenous (i.e. multiple bodies/materials as in over molded, or even liquid filled) and any combination thereof. Thus, the bushing  110  can have portions of greater and lesser density. The multiple material configuration allows a portion of the bushing  110  designed for contacting the compression tube  22  to be made of a complimentary non degrading material, while a supporting portion of the bushing is made of a less expensive material. Similarly, the materials can be chosen for performance such as an underlying portion of the bushing  110  being relatively resilient—deformable, while the surface coating provides a lubricious interface with the compression tube  22 . 
         [0058]    The bushing  110  can be formed from a variety of materials which provide the necessary radial expansion upon axial compression, along with the necessary wear characteristics and resilience. Further, the bushing  110  is sufficiently resilient to functionally return to an uncompressed (un-radially expanded) configuration upon the removal of a longitudinal compressive force between the piston head  70  and the piston body  90 . The bushing  110  can be a polymer material including but not limited to nylon or PTFE coated polymers including nylon. A representative material for the bushing  110  is a polymer, such as but not limited to polyurethane. The specific material of the bushing, polymeric or metal, is determined by the intended operating parameters of the compression piston  60  and airgun  10 . Thus, the bushing  110  can be non-metal. 
         [0059]    The relative size and/or weight between the piston body  90  and the piston head  70  can be selected to be between approximately 1:20 to 20:1. That is, depending on the intended operating characteristics, materials and design parameters, the piston body  90  can be 95% of the length of the compression piston  60  and the piston head  70  can be 5% of the length of the compression piston  60 . Conversely, the piston body  90  can be 5% of the length of the compression piston  60  and the piston head  70  can be 95% of the length of the compression piston  60 . 
         [0060]    Alternatively, it is contemplated the ratio of the weight (or mass) of the piston body  90  to the piston head  70  can be selected to be any of a variety of ratio from approximately 20:1 to 1:1 to 1:20, depending on the intended operating characteristics, materials and design parameters. 
         [0061]    Similarly, depending on the intended operating characteristics, materials and design parameters, the bushing  110  can be approximately 1% of the length to approximately 95% of the length of the compression piston  60 . Further, again depending on depending on the intended operating characteristics, materials and design parameters, the weight (or mass) of the bushing  110  can be selected to range from approximately 1% to 95% of the weight (or mass) of the compression piston  60 . 
         [0062]    The spring  50  can contact or engage the compression piston  60  at any of a variety of locations. For example, the spring  50  may contact piston head  70  directly, the capture piece  84 , such as the capture nut or bolt, the bushing  110 , the piston body  90 , or any combination thereof. 
         [0063]    The orientation of the spring  50 , such as a gas spring, is independent of the compression piston  60 . That is, the gas spring piston  58  of the gas spring or the gas spring body  52  of the gas spring can contact the compression piston  60  for selectively moving the compression piston between the first and the second positions, such as from the cocked position to the fired position. 
         [0064]    The interaction of the compression piston  60  and the compression tube  22  during firing of the airgun  10  is selected to reduce recoil/vibration, increase efficiency of the airgun using a spring to move the compression piston (contacting the gas spring) relative to a compression tube. 
         [0065]    As set forth above and referring to  FIGS. 13-15 , the spring  50  causes the compression piston  60  to move from the cocked position to the fired position within the compression tube  22 . As the compression piston  60  moves from the cocked position to the fired position under bias from the spring  50 , air in front of the piston head  70  (and seal  80 ) compresses in the compression tube  22  and the pressure rises. As the pressure ahead of the piston head  70  in the compression tube  22  rises, the piston head begins to decelerate. The inertia of the piston body  90  continues forward toward the transfer port  24 . The deceleration of the piston head  70  and the inertia of the piston body  90  changes the relative longitudinal spacing of the piston head and the piston body and simultaneously longitudinally compresses the bushing  110  while still driving the compression piston  60  toward the end of the compression tube  22  toward the transfer port  24 , forcing the high pressure air through the transfer port (and into the barrel  12  of the airgun  10 ). The longitudinal compression of the bushing  110  forces the bushing outward (radially expands) to contact the inner wall or surface  23  of the compression tube  22 , thereby acting as a braking system. The slightly longer deceleration time of the piston head  70 , as the piston body  90  compresses the bushing  110 , allows more air to flow through the transfer port  24  (into the barrel  12 ) so as to add energy to the projectile, and the energy is no longer available to contribute to reversal of the direction of travel of the compression piston  60  within the compression tube. As pressure continues to rise in the compression tube  22  ahead of the piston head  70  and the piston head slows to a stop, the bushing  110  is at full compression and exerting its maximum force into the compression tube, the engagement of the bushing and the compression tube resists backwards travel of the compression piston  60  and piston head  70 . This reduction in backward travel of the compression piston  60  including the piston head  70  keeps the volume between the front of the piston head (the seal  80 ) and the front end of the compression tube  22  (volume to the transfer port  24 ) low, thus maintaining higher compression tube pressure for a longer period of time, allowing an additional amount of energy to be added to the projectile. 
         [0066]    The removal of the longitudinal compression on the bushing  110  allows the bushing to return to the uncompressed state and the longitudinal spacing of the piston head  70  and piston body  90  returns to the non firing state. 
         [0067]    The amount of radial expansion of the bushing  110  can be influenced by the profile of the contacting surfaces of the bushing  110  and the piston body  90 . For example, referring to  FIGS. 8 ,  9 ,  11  and  12 , the front end  96  of the piston body  90  includes a taper or wedge surface  94  and the corresponding surface of the bushing  110 , such as the rear end  118  or inner wall  114  includes a taper  111 . Thus, upon the piston head  70  decelerating first and the longitudinal distance between the piston body  90  and the piston head reducing, the contacting inclined surfaces of the piston body and the bushing  110  tend to splay the bushing against the compression tube  22 , thereby resisting rearward motion of the compression piston  60  and increasing the mass to compressed air passed through the transfer port  24  to the barrel  12 . 
         [0068]    The action within the components of the compression piston  60  and interaction with the compression tube  22  during movement of the compression piston from the cocked to the fired position also reduces system vibration and recoil (1) by decreasing the rate of deceleration (jerk) (2) by isolating the forward metallic portion of the piston body  90  from contacting the wall  23  of the compression tube  22 , and (3) by damping the oscillation of bounce back of the compression piston. 
         [0069]    Thus, in one configuration, the piston body  90  is moveable relative to the piston head  70  along the longitudinal axis to the extent the bushing  110  is compressible along the longitudinal axis. However, it is understood that as the longitudinal spacing for receiving the bushing  110  can be adjusted, the amount of longitudinal displacement of the piston head  70  relative to the piston body  90  can be greater, equal to or less than the amount of longitudinal compression of the bushing under operating parameters. 
         [0070]    Thus, the present disclosure provides a method of using the spring  50  to urge the compression piston  60  to move within the compression tube  22  towards a muzzle end of the compression tube, the compression piston including the piston head  70  and the piston body  90 , the piston head being longitudinally displaceable relative to the piston body; and radially expanding the bushing  110  intermediate the piston head and the piston body a sufficient radius to decelerate at least a portion of the compression piston relative to the compression tube. The sufficient radius can be sufficient to contact the bushing  110  with the inner surface of the compression tube. It is further contemplated the spring  50  can be a coil spring or a gas spring. The gas spring can includes the gas spring body  52  defining the sealed interior chamber  54  containing the compressed gas and the gas spring piston  58  extending into and moveable relative to the sealed interior chamber, the interior chamber retaining the compressed gas when the gas spring piston moves. 
         [0071]    Further, the method includes disposing the radially expandable bushing  110  on the compression piston  60 , wherein the compression piston has a piston head  70  including the piston seal  80  and the piston body  90 , and the bushing is intermediate the piston head and the piston body, the bushing expanding a sufficient radius to contact an inner surface of the compression tube  22  of an airgun  10  in response to longitudinal displacement of the piston head relative to the piston body; locating the compression piston  60  and the bushing  110  at least partially within the compression tube of the airgun; and mounting the spring  50  in the airgun to urge the compression piston to move within the compression tube towards a barrel or muzzle end of the compression tube. 
         [0072]    The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.