Abstract:
Improved devices for adapting revolvers for shooting practice with pellets. Advantages include qieter operation, no need of accesories for loading or unloading the primer and a universal size for cylinders of different lengths. A venturi passage separates the detonaing chamber from the bore, thereby accelerating and depressurizing the expansion gases and reducing noise. In one embodiment, the adaptor includes an internal cannon which is longitudinally displaced together with the pellet by the propulsion gasses. Improved adaptors are disclosed for pistols, for selfcentring a barrel liner and protecting rifling grooves.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is a continuation of application Ser. No. 08/888,939. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is related to shooting practice with firearms and concerns a device for enabling large-calibre firearms to be fired with small-calibre missiles or ammunition. The invention particularly concerns an adaptor device for a large-calibre gun for using low-cost and freely-available munition such as air-pellets and pistol primers, utilizing the thrust of the primers to safely fire pellets with a similar degree of accuracy and range but much less kinetic energy in comparison with the standard high-calibre ammunition rounds.  
           [0003]    The adaptor devices of the invention are devised for use on large-calibre guns in places not specially protected as shooting ranges, for instance. Among other uses, the invention allows indoor practice or teaching with rifles, pistols and revolvers, as well as handling collection-item or antique firearm pieces without wearing the barrel out.  
         BACKGROUND OF THE INVENTION  
         [0004]    Practice with large-calibre firearms in times of peace is restricted to shooting ranges, whereto the shooter has to travel with all the precautions, expense and bother this implies. By no means should firearms such as 9 mm pistols or .38 or .357 Magnum revolvers, for example, loaded with standard-calibre ammunition be fired at home, be it for reasons of practice, training or learning, because of the serious risk of death or injury to the household and the neighbourhood as well as because of the loud noise these guns make when going off.  
           [0005]    For this reason, the guns one may practice with without leaving ones&#39; houses are of the compressed-air type, using 4.5 or 5.5 mm pellets as ammunition and even then taking care. However, using compressed-air guns for shooting practice is a rather poor substitute for those who wish to learn or become abler with a firearm of a larger calibre. The use of compressed air instead of gunpowder and the differences in weight, size and blast backlash between one kind of gun and the other significantly vary the responses and feel one gets from handling guns.  
         BRIEF REVIEW OF THE PRIOR ART  
         [0006]    The prior art has indulged heretofore in attempting to develop suitable adaptor means using readily available pellets for reduced-calibre missiles and primers as detonators.  
           [0007]    U.S. Pat. No. 4,361,093 to Michael Saxby dicloses an adaptor device for firing pellets with a larger-calibre firearm, such as a pistol, revolver or rifle, using primers. The adaptor comprises a two-part ammunition round including a casing externally shaped as a .38 or larger-calibre round of ammunition for loading in a .38 or larger-calibre pistol or revolver in the same way as an ordinary round of ammunition. The casing has a longitudinal passage housing a tubular insert. A tapered socket is provided behind the insert at the base of the casing for holding a 4.5 mm small pistol primer. The forward end of the insert forms a seat for a .177 or .22 pellet in the casing passage. Saxby suggests that the diameter of the forward portion of the passage be slightly larger than the nominal pellet diameter but slightly smaller than the overall maximum diameter of the pellet, i.e. of the pellet skirt. A narrow flame duct extends through the insert for conveying and moderating the thrust of the primer blast on the pellet when the trigger is pressed and the firing pin of the firearm strikes the primer.  
           [0008]    In some of Saxby&#39;s embodiments for pistols and revolvers, the adaptor device further includes a removible barrel liner which is inserted through the muzzle of the gun. The liner is a tubular member provided with an O-ring or resilient means at the breech end and clamping means at the muzzle end, the liner small-calibre bore abutting and naturally aligning with the casing chamber. Suggested clamping means is a split ring or plural collets associated with a threaded fastener which screws on to an extension of the liner. Another embodiment of the adaptor device, for 5.56 mm calibre rifles, is designed without the barrel liner.  
           [0009]    The socket is dimensioned for the primer to snugly fit therein, meaning that the primer has to be push-fitted into the socket. In some of Saxby&#39;s embodiments, the socket is formed in the base end of the insert and both are supplied together as a unit. The gun is loaded by first push-fitting the pellet and then the insert forwards into the casing chamber, avoiding the risk of the user pushing primers into position. In another embodiment, the primer is seated against the base end of the insert and the loaded casing may be supplied as a unit.  
           [0010]    U.S. Pat. No. 4,735,009 to Thomas Jett dicloses an adaptor kit which allegely simplifyies Saxby&#39;s cartridge casing. The casing is a one-piece insert including a pellet seating on the nose end of the casing passage. This enables loading the pellet through the nose end of the cartridge. Tools are disclosed for assisting in loading the primers and pellets into the adaptor.  
           [0011]    Jett further discloses a liner for mounting with O-rings inside the bore of the hand-gun barrel. Substantial lubrication of the bore and the O-rings is recommended before inserting the liner in the barrel.  
           [0012]    In all the above-cited art, the primer seat is dimensioned for the primer to fit snugly, that is, a certain amount of pushing force is required to place the primer in its proper position. This represents certain drawbacks, not the least of which is the risk of the primer detonating under manual pushing pressure. In fact, Jett suggests placing the primer on a flat surface and pressing the base orifice of the cartridge on to the primer, risking explosion and bodily danger such as loosing a finger.  
           [0013]    Moreover, only primers of a predetermined size may be used with such adaptor devices. Specifically, 5.5 mm large-pistol primers may not be used with a device designed for 4.5 mm small-pistol primers.  
           [0014]    There is also the risk of the primer falling out of the seat and the difficulty in removing a spent primer remains. Jetts&#39;s &#39;009 patent suggests a gripping tool for facilitating reloading a primer. It is likewise difficult to safely disarm an unused primer using an inertial bullet puller.  
           [0015]    Jetts&#39;s &#39;009 patent is forced to locate the pellet seat right in the nose of the casing, meaning that the pellet skirt is deformed at loading. At firing, the pellet has just begun to accelerate when it leaves the casing. In an application in a revolver, the liner rear end may not abut the cartridge nose end so that firing power is lost as the detonation gases leak out of the liner.  
           [0016]    In pistol applications, the liner abuts the cartridge casing and the latter is of exactly the same length as a standard-calibre ammunition round. There is thus the danger of loading and attempting to fire real ammunition with the liner accidentally left in the barrel, with disastrous consequences. In revolver applications, the ammunition rounds are of fixed length and thus suitable only for revolvers with cylinders of that size; otherwise a gap is left between the forcing cone or tapered breech end of the liner and the casing nose such that the speed of the pellet is rather low to insure that it crosses the gap properly.  
           [0017]    Neither are the prior art liners adaptable to gun barrels of different lengths. I also believe that the liner in Jetts&#39;s &#39;009 patent may not withstand explosive pressures and thus be displaced forward during firing. This is because the lubricated O-rings either run easily along the barrel bore such that the liner shifts easily during firing or they are sized so that they press hard enough thereagainst to avoid shifting but making the liner harder to insert.  
           [0018]    The external dimensions of the adaptor cartridges of the prior art are designed to be identical to a standard pistol ammunition round. There is thus a danger in backfiring live ammunition with the liner accidentally in the barrel.  
           [0019]    Other known art but further removed from the present invention include U.S. Pat. No. 3,952,657 to Hans-Joachim Kaltmann, U.S. Pat. No. 4,610,485 to Roland Bertiller and U.S. Pat. No. 4,682,545 to Thomas Jett.  
         SUMMARY OF THE INVENTION  
         [0020]    A main object of the present invention is an economical and easy-to-use solution to enable both beginners and expert sharp-shooters to practice with large-calibre firearms outside shooting ranges and other special installations and inside inhabited environments such as in ones&#39; own home.  
           [0021]    Another object of the invention is to provide simple means for firing a large-calibre gun with lightweight munition, for reasons of instruction, practice, competition or for using antique gun-pieces without wearing the barrel down, thus preserving its value as a collection item.  
           [0022]    And another object of the invention is to provide a means for shooting practice with large-calibre firearms using low-cost and freely-available ammunition, using detonating means such as a primer having a detonating power substantially less than that of a standard ammunition round.  
           [0023]    An important object of the present invention is a reduced calibre adaptor device which eliminates or substantially reduces the risk of manipulating primers.  
           [0024]    A further object is an adaptor device with means for seating a primer, wherein the latter may not fall out but which the remains thereof are easy to remove once spent or while still active, in particular without recourse to accessory tools.  
           [0025]    And another important object is an adaptor device suitable for primers of different sizes.  
           [0026]    An additional object of the invention is a barrel liner including simple and easy-to-use means to secure and keep it in place during firing.  
           [0027]    Yet another object of the invention is to preclude the risk of firing real ammunition in a pistol or rifle with the adaptor liner left in the barrel.  
           [0028]    A further object of the invention is an adaptor device which may be used with revolvers of different length cylinders.  
           [0029]    Yet a further object of the invention is an adaptor device providing a hermetic seal in revolvers.  
           [0030]    Another object of the invention is to improve the the performance of the adaptor cartridge, in particular providing a retractible insert for increasing the thrust on the pellet.  
           [0031]    Another object of the invention is to reduce the noise heard at primer detonation.  
           [0032]    The present invention essentially comprises an adaptor device. The adaptor comprises a cartridge casing adapted for holding a pellet and a primer in prearranged positions. The adaptor includes an elongated tubular casing having an external shape generally approximating or replicating the standard calibre ammunition round. As defined in claim  1 , the casing has: a seat for the primer provided at the casing rear end; a base removibly attached to said casing rear end to retain the primer in the seat, said base provided with an orifice for passage of the firing pin therethrough; a longitudinal bore of a diameter which is that of the reduced calibre, the bore extending from the casing nose end towards a position inside the casing intermediate the rear and nose ends; an inward rim forming a missile seat at the rearward end of the bore and a narrow passageway for passing expansion gases generated by the primer detonating to the missile to propel the munition out of the bore and the firearm barrel.  
           [0033]    The base is preferibly screwable onto the casing rear end after a primer has been placed in the primer seat and unscrewable off the casing rear end to discard spent primer reamins. Other preferred forms are defined in dependent claims.  
           [0034]    In one preferred embodiment, the adaptor device further includes a barrel liner adapted to be pushed into the gun barrel through the muzzle end thereof until it abuts against the nose end of a casing loaded in the chamber. The liner has an external thread partly protruding out of the gun barrel mouth at the muzzle end, a sleeve made from a deformible plastics material and which covers a part of the liner tube after the thread and a nut for screwing onto the thread to tighten against the sleeve until the sleeve expands diametrically to press against the internal wall of the barrel, thereby inmobilizing the liner tube. The liner has at least one O-ring housed in a respective circumpherencial groove adjacent to the rear end of the tube to keep it centred inside the barrel and maintain a gap along the length between the tube and the barrel.  
           [0035]    In a particularly preferred embodiment, the casing houses a longitudinally displaceable cannon containing the longitudinal bore for loading the reduced calibre missile and spring means for urging the displaceable cannon towards the casing rear end, the nose end of the casing including an orifice sixed to enable the cannon to emerge therethrough under the effect of expansion gasses produced by a detonating primer struck by the firing pin. The liner rear end may further be made of a relatively soft plastics material for initially absorbing strikes from a displaceable cannon in the casing until the material becomes sufficiently gorged to abut the cannon in a maximum displaced position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    The above-stated and other novel features and aspects of this invention and how it may be reduced to practice may be understood better from the following detailed description of a preferred embodiment shown in the attached drawings, wherein:  
         [0037]    [0037]FIG. 1 is a longitudinal-cross section of an adaptor device, according to this invention, for a cylinder-type gun, such as a revolver or some large-calibre rifles.  
         [0038]    [0038]FIG. 2 is a longitudinal-cross section of an adaptor device, according to this invention, for a fixed-chamber gun, such as a large-calibre pistol.  
         [0039]    [0039]FIG. 3 is a longitudinal-cross section of a barrel of a pistol showing the adaptor device therein, according to an improved embodiment of the invention.  
         [0040]    [0040]FIG. 4 shows a liner tube of an adaptor device for a revolver, according to an improved embodiment of the invention.  
         [0041]    [0041]FIG. 5A shows a first embodiment of a rear tube of the adaptor device for the revolver of FIG. 4. FIG. 5B illustrtaes the embodiment of FIG. 5A in a firing position.  
         [0042]    [0042]FIG. 6 shows a second embodiment of a rear tube of the adaptor device for the revolver of FIG. 4.  
         [0043]    [0043]FIG. 7 shows a third embodiment of a rear tube of the adaptor device for revolvers.  
         [0044]    Finally, FIG. 8 shows a fourth embodiment of a rear tube of the adaptor device for revolvers.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0045]    Describing in detail how the above-stated invention may be put to practice, FIG. 1 shows an embodiment of the adaptor device  11  of the present invention for a revolver, typically (the firearm itself is not depicted in the drawings). The adaptor  11  comprises a tubular body  13  internally traversed by a longitudinal bore  15 , preferibly with rifling grooves  17  for increased precision.  
         [0046]    The external diameter of the tube  13  is recessed at its forward part  19  by a step  21  for making a stop against the inlet mouth of of the gun chamber. More precisely, the diameter of the forward part of the tube  13 , between the step  21  and the outlet end  23 , is that of the standard gun ammunition or bullet whereas the external diameter in the forward part  25  corresponds to the ammunition cartridge.  
         [0047]    The rear end of the tube  13  itself is open, internally recessed and threaded for attaching a primer-holder  27  provided with a base  29  which continues in a circumpherencial flange  31 . The base  29  is centrally traversed by a hole  33  extending towards a seat  35  for the primer  37 , any of which marketed as “small pistol” o “large pistol” primers may be used. This seat  35  comprises a rear conical cavity, having a diameter decreasing towards the hole  33 , and a forward cylindrical cavity, joined to one another, the latter closed in part by a primer retainer  39  formed by a cylindrical member having a central passage  41  coaxially aligned with the inlet end of the bore  15 .  
         [0048]    In an application of the adaptor  11  for .44-calibre Magnum revolver, as it is popularly called, the full length of the unit  11  is 41.9 mm, broken down as: 7.8 mm forward length  19 , 32.6 mm rear length  25  and 1.5 mm width of the flange  31  at the base  27 . The diameters of these three parts are 10.9 mm and 11.6 mm for the front  19  and rear  25  tubular parts, respectively, and 13.1 mm for the base  29 , whereas the diameter of the bore  15  is 4.5 or 5.5 mm according to the ammunition  43  to be used, the diameter of the gas passage hole  41  may be 1 mm and that of the seat  35  according to the size of the primer  37 .  
         [0049]    To load the adaptor  11 , first the three members  13 ,  27  and  39  are disassembled, unscrewing the base  27  from the tubular body  13  to uncover and take out the primer-latch  39 . A conventional pistol primer  37  is placed on the seat  35  through the forward mouth of the base  27  and a pellet  43  or round shot, also conventional, such as of 4.5 or 5.5 mm, is placed in the chamber of the bore  15  through the rear mouth of the latter. The cylindrical primer-latch  39  is replaced again to separate the pellet  43  from the primer  37  and the base  27  is screwed back on to the tube  13 .  
         [0050]    The cylindrical member  39  is for retaining the primer  37  in its seat  35  where it may be struck by the firing pin of the gun through the hole  33  when the shooter presses the trigger (not shown). The expansion gasses produced by the detonation flow through the passage  41  and drive the pellet  43  through the latter, thereby producing the shot.  
         [0051]    The basic structure of the embodiment of FIG. 1 requires some changes when the adaptor device of the invention is for use with a gun having a chamber, as is the case of a pistol, for example a .45-calibre ACP pistol. The barrel  75  of the pistol is schematically drawn in FIG. 2 and is internally occupied in its entirety by the adaptor  81  of the present invention. The adaptor  81  comprises a liner tube  83  and a rear base  85 . The latter is internally threaded for screwing on the primer-latch  87  which, in turn, is provided with a passage  89  for gasses recessed at its forward part to form a pellet-housing chamber  91 .  
         [0052]    The adaptor  81  is pushed into the barrel  75  through the chamber  77  until its outlet-end mouth  93  protrudes through the outlet mouth  79  of the barrel  75 , revealing an external thread  95  for affixing a nut  97  for holding the adaptor  81  in place. The tube is further provided with an external circumpherencial groove for housing an O-ring  99  to hold the adaptor  81  fast inside the barrel  75 , specially during firing, thereby assisting in the aim of the shot.  
         [0053]    The entire length of the adaptor  81  is at least 127 mm, 11.4 mm of which correspond to the base, the diameter of which is 12 mm. The middle part of the tube  83  which goes inside the barrel  75  of the pistol has a diameter slightly less than 11.4 mm.  
         [0054]    A more preferred embodiment of the invention is set forth in FIG. 3 which schematically shows the barrel  75  of the pistol internally occupied by the adaptor  181  of the present invention. The adaptor  181  comprises an elongated liner tube  183  and a rear tube  187 , shorter than the former, screwably closed behind by a base  185 . The latter contains a housing for a primer  37 . The shape of the combination of the base  185  and the rear tube  187  is similar to a standard-calibre ammunition and includes a circumpherencial recess  201  abutting against a circumpherencial step in the chamber  77  which normally retains a standard ammunition cartridge.  
         [0055]    However, the overall length of the combination  185 - 187  is advantageously shorter that of standard-calibre ammunition as a means of protection against neglecting to take the liner  183  before shooting with real ammunution.  
         [0056]    The rear tube  187  includes the gas passage  89  recessed at its forward part to form a pellet chamber  91 . Gasses coming from a just-fired primer  37  are accelerated and decompressed through this passage, taking advantage of Venturi&#39;s principle to provide a silencer.  
         [0057]    The liner tube  183  is pushed into the pistol barrel  75  through the outlet mouth  79  until it abuts against the rear tube  187  loaded in the chamber  77 . The oulet mouth  193  of the liner tube is externally threaded  95  and in part protrudes out of the mouth of the gun barrel  75 . A nut  197  is then screwed onto the thread  95  to press against a sleeve  203  made from a deformible plastics material and which covers a part of the liner tube  183  after the thread  95 . The nut  197  is tightened until the sleeve  203  expands diametrically to press against the internal wall of the barrel  75 , thereby immobilizing the liner tube  183 .  
         [0058]    The liner tube  183  has two O-rings  199  housed in respective circumpherencial grooves adyacent to the rear end of the tube  183  to keep it centred inside the barrel  75  and, more importantly, maintain a gap between the metallic wall of the tube and that of the barrel  75  which preserves the grooves inside the latter.  
         [0059]    [0059]FIG. 4 shows the liner tube  183 ′ adapted to revolvers for a quieter shot. No accesory tools are needed for loading and unloading the pellet  43 . In this embodiment, the second O-ring  199  is replaced by a plastics rear end embodied by a short length of tube  205  made of a material known as “ DELRÍN ”. The length of the liner tube  183 ′ is such that the rear end  205  is at a scarce distance, advantageously in the order of a few tenths of a millimetre, from the forward end of the cylinder of the revolver (not illustrated), practically abutting agains the forcing cone of the revolver barrel when the liner tube  183 ′ is inside the revolver barrel and immobilized by the nut  197  expanding the sleeve  203 .  
         [0060]    The elongated liner tube  183 ′ of FIG. 4, which stays fixed inside the gun during a firing round, is complemented by a second unit of the adaptor device  181 ′ of the invention, formed by a short tubular member  11 ′ having an external shape and size similar to standard-calibre ammunition for that gun, as illustrated in FIGS. 5 and 6. This short tubular member  11 ′, or “pseudobullet” as it could be termed, loads the pellet  43  and the primer  37  for each shot. That is to say, in contrast to the forward tubular member  183 ′, the rear tubular member  11 ′ is changed for each shot, by the automatic turn of the cylinder, and has to be reloaded before it may be used again. It may be seen that both members  183 ′ and  11 ′ of the adaptor  181 ′ of the invention carry out different functions in that, respectively, one downscales the dimensions of the gun barrel whereas the other loads ammunition of another calibre.  
         [0061]    Describing in detail the embodiment of the tubular member  11 ′ shown in FIG. 5, it essentially comprises a tube  13 ′ having an external diameter which is recessed at its forward part  19  by a step  21  capable of abutting against the chamber inlet mouth of the gun. More precisely, the diameter of the forward part of the tube  13 ′, between the step  21  and the outket end  23 ′, is that of the bullet or standard ammunition for the gun whereas the external diameter in the forward part  25  corresponds to the ammunition cartridge.  
         [0062]    The rear end of the tube  13  itself is open, internally recessed and threaded for attaching a primer-holder  27  provided with a base  29  which continues in a circumpherential flange  31 . The base  29  is centrally traversed by a hole  33  extending towards a seat  35  for the primer  37 , generally any of which marketed as “ SMALL PISTOL”, “SMALL RIFLE”, “LARGE PISTOL ” or “ LARGE RIFLE ” primers may be used.  
         [0063]    The interior of the cannon  207  includes a bore  15  extending towards the forward end  23 ′ of the rear member  11 ′ and dimensioned for housing the pellet  43 . The bore  15  is communicated backwardly with a pasaje  41  coaxial therewith. The bore  15  of the barrel has a recess  208  to reduce the diameter of the rear part to compensate for the barrel moving forward against the plastics end  205  before the pellet exits. The recess  208  further stops the primer from passing through into the pellet chamber under the firing effect, specially when primers  37  of the “ SMALL ” type are used.  
         [0064]    More precisely, the opening in the forward end  23 ′ of the tube  13 ′ is big enough for the cannon  207  to be pushed forward and stick out of the tube  13 ′. The cannon  207  has a circumpherential shoulder  209  which does not let the tube get completely out of the tube  13 ′. Furthermore, a spring  211 , which is wound around the cannon  207  and has one end resting against the shoulder  209 , urges the cannon  207  inside the tube  13 ′.  
         [0065]    In order to load the rear member  11 ′, the base  27 ′ of the tube  13 ′ is unscrewed, a fresh primer is placed on the seat  35  and the base  27 ′ is placed back on again. A 4.5 mm pellet  43  or round shot, such as of the type marketed as “ GAMO ” or “ TORCAZ ” for example, is seated in the bore  15  through the forward mouth  23 ′ of the rear tube  13 ′ and the short tubular member  11 ′ is inserted in one of the cylinder receptacles, the gun ready for use. In this case, hollow cup-like or “ DIÁBOLO ” pellets may be used in spite of their greater structural fragility, since part of the energy generated by the gasses in expansion is consumed in forcing the cannon against the spring  211 , thereby reducing the power available for accelerating the pellet and, consequently, the destructive effect of the primer charge on this kind of ammunition which could otherwise become beheaded by an excess charge.  
         [0066]    When the shooter presses the trigger (not shown), the firing pin of the gun strikes the primer  37  in seat  35  through the hole  33 . The expansion gasses under the effect of the detonation propel the cannon  207  together with the ammunition  43 , compressing the spring  211 . Part of these gasses cross through the passage  41  and simultaneously accelerate the pellet  43  inside the cannon  207 . The cannon  207  is propelled out of the tube  13 , through the hole  23 ′ and strikes against the “ DELRIN ” material  205  of the long tube  183 ′. In this manner, a closed system is formed inside the liner tube wherein both the pellet  43  as well as the propellent gasses coming from the primer  37  are impelled. The pellet  43  shoots out of the little cannon  207 , thereby producing the shot, and the cannon  207  is retracted thereafter back into the tube  13 ′ again by the reaction of the spring  211 .  
         [0067]    With use, the cannon  207  repetitively strikes the plastics  205  and gradually wears it down with each shot until enough room is made to accomodate the little cannon  207  after some use. The inclusion of the displaceable cannon  207  provides compensation for different cylinder lengths found in revolvers of a same .38 o .357 MAG calibre, thereby providing a universal adaptor system for a given calibre.  
         [0068]    [0068]FIG. 6 illustrates an alternative embodiment of the rear tubular member  11 ″, which features a primer-latch  39 ′ included between the primer seat  33  and the chamber housing the cannon  207 ′. The primer-latch  39 ′ screws on to the same thread that attaches the base  27 ′. Apart from this, this embodiment of the tubular member posterior  11 ″ is similar to the one of FIG. 5.  
         [0069]    [0069]FIG. 7 ilustrates an alternative embodiment of the adaptor device of FIG. 1 which essentially consists of a rear tubular unit  151  which may operate as an adaptor alone or in combination with the forward tubular unit  183 ′ depicted in FIG. 4. The object of this embodiment is to replace the displaceable cannon  207  of FIGS. 5 and 6 with a tubular unit  151  of adjustable length, which the user may match to the length of the cylinder of his gun.  
         [0070]    It comprises a base  155  which, at its forward part, features an internal thread  213  for attaching an intermediate tube  215  including a first stretch  41 ′ of the gas passage. The intermediate tube  215  features in turn an internal thread  217  at its forward part for screwing on the short tube with adjustable penetration, the latter tube featuring the remaining parts of the gas passage stretch  41 ″ and of the bore  15 ′ for housing the pellet  43 . The set formed by the base  155  and the intermediate tube  215  have diametrical and longitudinal dimensions corresponding to a standard-calibre ammunition cartridge of the gun whereas the forward tip of the pellet-holder tube  157  looks like a bullet of this calibre.  
         [0071]    That is to say that the munition holder  157  is screwed into the intermediate tube  215  until the overall length of the unit assembled by the three members  155 ,  215  and  157  equals the length of the cylinder (a little less, in fact, to avoid jamming the cylinder). A counternut  219  is further assembled on the external thread of the munition-holder tube  157  and further tightened against the intermediate tube  215  to stop the joint from lossening.  
         [0072]    The base  155  is unscrewed from the intermediate tube  155  for loading the primer  37 . The intermediate tube  215  may be integrated into a single piece with the base  155  if a fully-cylindrical seat is provided, such that the primer  37  is held therein by the pressure exerted against the cylindrical sidewalls for enabling the primer  37  to be loaded from behind, through the firing-pin access hole  61 .  
         [0073]    Lastly, the alternative embodiment of the rear unit  151 ′ featured in FIG. 8 supresses the intermediate tube such that the length of the unit is not screwingly regulated. Rather, the length of the munition-holder tube  157 ′ is sized to the longest cylinder available on the market, such that the user should cut off the forward point  123  of the tube  157 ′ to the right size if the cylinder of his gun is shorter.  
         [0074]    Of course, changes, variations and aggregations may be made to any of the above-detailed embodiments, without departing from the scope nor the spirit of the invention. The same has been described by way of preferred embodiments, however those skilled in the art may suit it to other applications or introduce modifications without departing from the purview of the invention as set forth in the appended claims. For example, the base  29 ,  55 ,  85  may be press-fitted into the tube  13 ,  57 ,  83  instead of screwed onto the latter.