A caliber bar for locating the centerline of the bore at the muzzle of a gun including a frame, first and second trunnion housings axially mounted on the frame, each of the trunnion housings pivotally mounting three centering arms circumferentially spaced at 120.degree. from each other for contacting the side of a bore at axially spaced locations, and first and second liquid springs mounted on the first and second trunnion housings, respectively, for biasing the centering arms associated with each trunnion housing simultaneously outwardly equal amounts into engagement with the side of the bore, and detents for removably holding the first and second trunnion housings in the frame.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved caliber bar for determining 
the longitudinal centerline of the bore at the muzzle end of a gun. 
By way of background, after a gun or rifle has been fired a number of 
times, the bore at the muzzle end of a gun may be distorted so that its 
centerline no longer coincides with the alignment of the sights associated 
therewith. In the case of small caliber weapons capable of being carried 
by a person, several test rounds are fired at a target and the sights are 
adjusted to coincide with the line of fire. In the case of larger caliber 
weapons, various mechanical and/or optical devices have been developed for 
determining the centerline of the bore at the muzzle end of the gun. Some 
of these devices determine the bore centerline by direct measurement, 
others use indirect measuring means often determining a line parallel but 
not coaxial with the gun bore centerline. Devices, such as shown in U.S. 
Pat. No. 4,459,757, did not determine the centerline with accuracy within 
a short enough time period to be of use in the field. 
SUMMARY OF THE INVENTION 
It is accordingly one object of the present invention to provide an 
improved caliber bar for accurately determining the centerline of a muzzle 
bore, even though it may have been distorted in different ways, such as by 
belling out or by being off center in other respects. 
Another object of the present invention is to provide an improved caliber 
bar which can be operated by unspecialized personnel in the field with 
high accuracy and reliability. 
Still another object of the present invention is to provide a caliber bar 
which inherently can be used to find the centerline of bores of various 
diameters without requiring any adjustment or replacement of parts 
whatsoever. 
A further object of the present invention is to provide an improved caliber 
bar utilizing liquid springs which will force centering arms against the 
sides of the bore with sufficient pressure to maintain the caliber bar 
firmly in place during the alignment procedure. 
Still another object of the present invention is to provide an improved 
caliber bar utilizing liquid springs for biasing centering arms, the 
liquid springs being capable of providing high spring forces in the 
relatively small area in which the caliber bar is used. Other objects and 
attendant advantages of the present invention will readily be perceived 
hereafter. 
The present invention relates to a caliber bar for locating the centerline 
of the bore at the muzzle of a gun comprising a frame having a 
longitudinal axis, a first series of first arms spaced circumferentially 
from each other and movably mounted on said frame, a second series of 
second arms spaced circumferentially from each other and movably mounted 
on said frame, said first series of first arms being axially spaced from 
said second series of second arms, and biasing means for simultaneously 
biasing said first arms outwardly equal amounts from said longitudinal 
axis of said frame for engaging the side of said bore at a first location 
and for simultaneously biasing said second arms outwardly equal amounts 
from said longitudinal axis of said frame for engaging the side of said 
bore at a second location which is axially spaced from said first location 
to thereby cause said longitudinal axis of said frame to coincide with 
said centerline of said bore at the muzzle of said gun. 
The various aspects of the present invention will be more fully understood 
when the following portions of the specification are read in conjunction 
with the accompanying drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The improved caliber bar 10 includes a frame or outer housing 11 in which 
trunnion housings or bases 12 and 13 are mounted. Trunnion housings 12 and 
13 are identical in all respects except that housing 13 includes a 
threaded bushing 14 mounted in end wall 15 thereof. The threaded end 17 of 
rod 19 is screwed into bushing 14. Rod 19 includes an enlarged portion 20 
having an annular shoulder 21 which bears against washer 22 so that 
housing 23 bears against flange 24 of frame 11 when threaded end 17 is 
screwed into bushing 14. An optical collimator 25, which provides a 
reticle image (not shown) of the view along the centerline of rod 19, with 
overlaid grid, usually including cross hairs, is mounted at the end of rod 
portion 20. Rod 19 is coaxial with the longitudinal centerline 27a of 
frame 11. 
As noted above, trunnion housings 12 and 13 are identical except for the 
above described difference. Therefore unprimed numerals will be used to 
designate the various parts of housing 13, and primed corresponding 
numerals will be used to designate corresponding parts of housing 12, 
thereby obviating the necessity for redundant descriptions. 
Trunnion housing 13 includes a central portion 27 having three slots 29 
(FIGS. 2 and 5) extending longitudinally thereof and circumferentially 
spaced 120.degree. from each other. An end plate 15 is located at the end 
of central portion 27, and as noted above, end plate 15 differs in the 
above-described manner from end plate 15' of housing 12. A base 30 is 
located at the opposite end of central portion 27 from end plate 15. Base 
30 is of generally circular cross section (FIG. 5) except that it has 
cutouts 31 therein. Base 30 also includes longitudinally extending slots 
32 which are continuations of slots 29. 
The outer dimensions of plates 15 and 15' and bases 30 and 30' of trunnion 
housings 13 and 12 fit within frame 11 with a substantially zero tolerance 
fit so that housings 12 and 13 can be slid into the position shown in FIG. 
2. End plate 15 of housing 13 abuts shoulder 26. Spring detent 28 includes 
a housing (not numbered) which is screwed into base 30, and it includes a 
spring biased plunger 36 which is received in a mating aperture in frame 
11, thereby removably holding trunnion housing 13 in position. 
A plurality of centering arms 33 have first ends 34 having bores 35 therein 
(FIG. 6) which receive axle pins 37 when the ends of centering arms 33 are 
mounted in trunnion relationship in slots 32 of base 30. Axle pins 37 have 
ends 39 which are received in associated bores 40 of base 30, with the 
central portions of axle pins 37 being received in bores 35 of the 
centering arms. The opposite ends of axle pins 37 are received in bores 
56. Set screws 41 hold axle pins 37 in position. The ends of axle pins 37 
proximate set screws 41 are internally threaded to receive threaded puller 
members (not shown) used to pull pins 37 out of their associated bores. 
Each of the arms 33 include a longitudinal rib 38 and a broad outer 
surface 46. 
Trunnion housing 13 includes a bore 42 which receives the cylinder 43 of 
liquid spring 44 in slidable engagement, with the centerline of bore 42 
and cylinder 43 being coaxial with centerline 27a of frame 11. Piston 45 
of liquid spring 44 includes a threaded extension 47 which is threaded 
into plug 49 which forms a part of base 30. Plug 49 is held in base 30 by 
set screws 50 which are backed up by additional set screws 51. An annular 
collar 52 is formed integrally with housing 43 so that it provides an 
annular shoulder 53 which abuts annular shoulder 54 (FIG. 2) of base 30 to 
define the limit of movement to the right of cylinder 43. A finger 55 is 
located at the end of centering arm 33 and it has a surface 57 (FIG. 6) 
which bears against annular shoulder 53. Thus liquid spring 44 will bias 
all three fingers 33 outwardly simultaneously equal amounts in a 
counterclockwise direction in FIG. 2 when collar 52 is not abutting 
shoulder 54. 
As noted above, the bore 59 at the muzzle end 60 of a gun will be enlarged 
with use. This enlargement may be a regular or irregular belling out or it 
may constitute other types of distortions which cause the centerline of 
the bore to have an alignment which departs from the original centerline. 
Furthermore, the path of the projectile will follow the centerline of the 
portion of the muzzle end of the bore which has a length roughly four to 
five times the nominal diameter of the bore. For example, if the bore is 
105 millimeter, the projectile will follow a path which is determined by 
the longitudinal axis of four to five times 105 millimeter at the muzzle 
end of the bore. 
The improved caliber bar 10 of the present invention allows precise 
alignment of the optical sight on the gun (not shown) with the centerline 
of the muzzle end of the barrel, with the projected centerline being 
visually observed through optical collimator 25. In this respect, the 
three sets of centering arms 33 will automatically find the center of the 
portion of the bore where their ends 61 of surfaces 46 contact the 
adjacent portions of bore 59. The ends 61' of centering arms 33' will also 
determine the center at the portions of bore 59 which they contact. It 
will be noted that both sets of arms 33 and 33' contact the muzzle end of 
the bore at locations spaced axially inwardly from the very end of the 
muzzle to thus accurately determine the bore centerline, because the very 
end of the muzzle cannot be used as a reference point for accurately 
determining the bore centerline. Centering arms 33 extend through slots 63 
in frame 11 so that they can move back and forth about the axes of pins 
37, as required. The ends 48 of arms 33 will abut the inner surface of 
frame 11 to further limit the counterclockwise rotation of arms 33 after 
shoulder 53 abuts shoulder 54. 
The geometry of the construction is such that centering arms 33 will be 
biased simultaneously equal amounts in a counterclockwise direction 
because of the interengagement between fingers 55 and collar 52. The same 
is true of centering arms 33' which may be biased outwardly equal amounts 
which may be different than those of arms 33. The foregoing being the 
case, the centerlines of cylinders 43 and 43' and the centerline of frame 
11 will coincide with the centerline of the portion of muzzle bore 59 
which is contacted by the outer curved ends 61 of centering arms 33 and 
33'. The collimator 25 is aligned with the centerline of frame 11, and 
thus will be aligned with the centerline of bore 59. Therefore when the 
collimator 25 has the target in its cross hairs, the centerline of the 
bore at the muzzle end of the gun will be in line with the target, and 
then the sights (not shown) on the gun can also be adjusted to be aligned 
with the target with any known variance between the bore centerline and 
the line of fire being taken into account, and thus the gun sights will be 
properly correlated with the centerline of the bore at the muzzle end of 
the gun. The contacting ends 61 and 61' of centering arms 33 and 33' are 
curved (FIG. 5) so as to make substantial point contact with bore 59. 
While the collimator 25 is a well known device, by way of broad 
description, it includes optics which essentially include a prism and lens 
structure so that when the end 25a is pointed at a target, the latter will 
be visible to a viewer looking down at surface 25b, and the target will 
appear on a suitable grid work or cross hairs at surface 25b. When the 
muzzle 60 has its centerline 27a pointed directly at the target, the 
latter will be centered directly on the cross hairs. 
While the foregoing description has specifically described the present 
invention in a preferred form utilizing liquid springs, it will be 
appreciated that other suitable types of springs may be utilized to 
achieve the broader concepts of the present invention. 
While a preferred embodiment of the present invention has been disclosed, 
it will be appreciated that it is not limited thereto but may be otherwise 
embodied within the scope of the following claims.