Breech/receiver assembly for automatic cannons

Abstract

A breech/receiver assembly for automatic cannon incorporates a rotating chamber and a recoil and counterrecoil buffer integrated within the breech. The breech, including the buffer means, recoils with the barrel and because the mass of the buffer is added to the breech and barrel, recoil forces transferred to trunions mounting the breech/receiver assembly are significantly reduced. The configuration of the breech/receiver assembly, including the rotating chamber and integrated recoil/counterrecoil buffer, enables a shorter receiver, thereby enabling a vehicle-mounted cannon utilizing the breech/receiver assembly of the present invention to elevate to high angles while still maintaining a relatively low gun silhouette.

Description

The present invention is directed to automatic cannon utilizing telescoped ammunition, wherein the shell projectile is entirely contained within the shell casing. More particularly, the present invention is directed to a breech/receiver assembly for such automatic cannon for enabling reduced recoil loading on gun mount trunions thereby facilitating the mounting of the automatic cannon on assault vehicles or the like.

BACKGROUND

Telescoped ammunition was developed in part to provide for greater shell storing densities and ease of ammunition feeding. These advantages stem from the fact that the telescoped shells are more uniform in diameter and shorter in length than conventional shells of comparable calibre. Although telescoped shells are generally larger in diameter than their counter-part conventional shells, their generally shorter length can facilitate shell handling and feeding.

Other advantages stemming from the shorter length of the telescoped ammunition include the cannon design and configuration. For example, the breech, or operating mechanism may be shorter. This is particularly true when a rotating type chamber is utilized. In turn, a shorter operating mechanism in a cannon may enable the cannon to elevate to high angles while at the same time providing a low profile for the cannon when mounted on an assault vehicle.

The present invention incorporates a rotating chamber within a breech/receiver assembly which is compact and lightweight to enable vehicle-mounted cnnons utilizing the breech/receiver assembly of the present invention to elevate to high angles. Additionally, the breech/receiver assembly provides for reduced recoil loading on gun mount trunions thereby facilitating the mouting of the cannon on the assault vehicles.

SUMMARY OF THE INVENTION

A breech/receiver assembly for automatic cannon in accordance with the present invention includes a receiver and a breech having means defining a first aperture therein for receiving a chamber and a second aperture therein communicating with the first aperture and adapted for receiving a barrel at an outside end thereof. The breech is adapted for slidable mounting within the receiver.

In addition, a chamber is provided with means for defining an opening therethrough sized for accepting ammunition. The chamber is mounted within the breech first aperture for rotational movement from a first position in which the opening therein is aligned with the breech second aperture and a second position in which the opening therein is aligned for simultaneously accepting ammunition from a point outside of the breech and ejecting ammunition casings therefrom.

Combination recoil counterrecoil buffer means is disposed within the breech for buffering barrel recoil and counterrecoil forces as the breech block slides within the receiver. In this manner, the buffer means is integrated within breech and moves with it under recoil and counterrecoil. Because this increases the moving mass of the cannon, that is, the buffer means is added to the breech and barrel mass and moves with it, the recoil forces transferred to the trunion mounts is significantly reduced.

Additionally, because of the integrated buffer, the breech/receiver assembly is shorter than conventional cannon which has the buffer disposed behind the breech. This reduction in the length of the breech/receiver assembly in accordance with the present invention thereby permits high angle elevation of the cannon while still maintaining low mounting vehicle profile, or silhouette.

More particularly, the breech/receiver assembly in accordance with the present invention has a breech which includes four longitudinal chambers therein, with the first and second of such longitudinal chambers being disposed on opposite sides of the breech aperture and having axes generally parallel with one another, and the breech second aperture longitudinal axis. Third and fourth of the longitudinal chambers within the breech are disposed on opposite sides of the breech first aperture and have axes generally parallel with the first and second longitudinal chambers. The combination recoil and counterrecoil buffer means disposed within the breech includes two fixed pistons, one piston being disposed in each of said first and second longitudinal chambers, and each having a piston rod attached to the receiver, and two movable pistons, one movable piston being disposed in each of the third and forth longitudinal chambers.

In fact, the combination recoil and counterrecoil buffer means in accordance with the present invention includes two separate buffers, one being disposed on either side of the chamber disposed in the breech first aperture. the buffer means of the present invention are not aligned with the barrel axis, however, are symmetrically disposed thereabout, thereby eliminating any twisting moment or torque, between the buffer means and the barrel upon recoil and counterrecoil. As hereinbefore mentioned, because the buffers are not conventionally placed behind the breech and in line with the barrel axis, the overall length of the receiver is significantly reduced.

Continuing, the breech/receiver assembly in accordance with the present invention has the first and third longitudinal chambers in fluid communication with one another, and the second and fourth longitudinal chambers in fluid communication with one another. Further, hydraulic fluid is disposed in all of the longitudinal chambers with the hydraulic fluid being disposed between a forward face of the fixed pistons and a forward face of the movable pistons with the fluid communication between the longitudinal chambers enabling hydraulic fluid to flow from the first longitudinal chamber to the third longitudinal chamber and from the second longitudinal chamber to the fourth longitudinal chamber upon recoil of the breech. The fixed pistons moving within the breech first and second longitudinal chambers forces the hydraulic fluid from the first longitudinal chamber into the third longitudinal chamber and from the second longitudinal chamber into the fourth longitudinal chamber. The buffer means includes an inert gas compressed within the second and fourth longitudinal chambers, between a rear end thereof and a backface of the movable pistons disposed therein. The inert gas is under sufficient pressure to move the movable piston forward after recoil of the breech and force the hydraulic fluid from the second and fourth longitudinal chambers and into the first and third longitudinal chambers respectively, thereby returning the breech to a battery position.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the present invention will be better understood by the following description and drawings in which:

FIG. 1 is a perspective view of a breech/receiver assembly in accordance with the present invention, as used with an automatic cannon, and including a gun mount for securing the cannon to an assault vehicle or the like. Also shown are gun mount trunions coaxially aligned with the chamber of the breech/receiver assembly and generally showing a short breach/receiver assembly for enabling high elevation of the cannon while maintaining a low profile or silhouette;

FIG. 2 is an exploded perspective view of the breech/receiver assembly generally showing the receiver, in phantom lines, a breech mounted for a slidable motion within the receiver, a rotating chamber and an integrated recoil and counterrecoil buffer means;

FIGS. 3a and 3b are cross-sectional side views of the breech/receiver assembly showing rotation of the chamber within the breech when the breech is at a battery position for enabling reloading of the chamber and ejection of casings; and,

FIGS. 4a, 4b, 4c and 4d are cross-sectional views of one of the recoil/counterrecoil buffers showing longitudinal chambers within the breech assembly and a fixed and movable piston therein, each of the figures showing a different position of the fixed and movable pistons within the longitudinal chambers as the cannon undergoes recoil and counterrecoil.

DETAILED DESCRIPTION

Referring now to FIGS. 1 and 2, a breech/receiver assembly 10 in accordance with the present invention generally includes a receiver 12, a breech 14, a chamber 16 and combination recoil and counterrecoil buffer means 18 disposed within the breech 14.

The receiver 12 is generally rectangular in shape and has a pair of coaxial trunions 22 for pivotally mounting the breech/receiver assembly 10 to upstanding portions 24 of a gun mount 26.

Shown in phantom lines in FIG. 2, tracks 28 are provided on inside surfaces 30 of the receiver 12 for slidably mounting the breech 14, which is adapted for such slidable mounting within the receiver by means of grooves 32 in the sides 34 thereof. An opening 40 in the top 42 of the receiver (FIG. 1) and another in the bottom of the receiver (not shown) enable access to the breech 14 and the chamber 16 therein when the breech is in a battery position for the loading of shells and ejection of casings as will be hereinafter discussed in greater detail.

As more clearly showin in FIG. 2 the breech 14 has a first aperture 44 therethrough from the top 46 to the bottom 48 of the breech. The aperture 44 is generally rectangular and is disposed rearwardly of the breech 14 center and has therewithin the chamber 16 pivotally mounted by a pair of sidewardly projecting trunions 54 which, when the breech is in a battery position, are aligned with the axis of the receiver mounting trunions 22. This feature enables elevation of the gun without axially displacing the chamber from shell feeding apparatus (not shown) disposed beneath the receiver and within an assault vehicle (not shown) on which the breech/ receiver assembly 10 is mounted.

A pair of trunion plates 58 having holes 60 therein for receiving the chamber trunions 54 support the chamber 16 within the breech first aperture 44 and fit downwardly into recesses 62 on opposite sides of the breech first aperture 44.

A second aperture 66 communicating with the first aperture 44 is formed in the breech 14 and adapted, by means of threads 68, for receiving a barrel 70.

Chamber rotating means, or apparatus, 72 are provided for rotating the chamber, which generally includes a hydraulic cylinder 74, mounted to the receiver 12 by means of a bracket 75, a slide 76 and an arm 78.

The cylinder 74 is mounted within the receiver and over the breech in position for engagement with the slide 76. Slide 76, is mounted on a track 79 formed on the breech top 46. It should be appreciated that two tracks 79 are shown and the chamber opening apparatus 72 may be mounted on either side of the chamber, or on both sides. The arm 78 interconnects the slide 76 with the chamber 16 by means of a pin 80 engaging a hole 82 in the chamber 16 and an opposite end 84 of the arm engages a bracket 86 on the slide 76 by means of a pin 87.

A latch 88 is disposed beneath the slider 76 for engagement with a notch 90 formed in the track 79 and a lock 92 is spring mounted to a top 93 of the slide 76 by means of a pin 93a and spring 95.

In operation, the chamber is locked in the battery position by means of the latch 88 engaging the notch 90 by means of a lower lip 88a. Upon actuation of the hydraulic cylinder 74, a piston end 96 thereof, engages the lock 92 pushing it against the spring 95, enabling the latch 88 to be released from the notch 90. At this point, the slide 76 is free to slide rearwardly along the track 79 and rotate the chamber 16 open by means of the arm 78.

When the latch 88 is raised from the notch 90, an upper lip 88b thereof engages the piston end 96, thereby fixing the piston end to the slide 76 when the latch is not engaging the notch 90 and linking the piston end 96 to the chamber 16. This enables the hydraulic cylinder 74 to pull the slide 76 forwardly to close the chamber 16.

Closing of the chamber 16 is caused by retracting the hydraulic cylinder 74 enabling the slide to move forward along the track 79 thereby causing the closing of the chamber 16 and locking of the chamber to the breech 14 as the latch 88 re-engages the notch 90. The piston end 96 is not attached to the slide, at this time, hence, the breech is free to slide within the receiver 12 upon recoil of the gun without further movement of the cylinder 74.

The chamber 16 has an opening 94 of appropriate size, or calibre, for accepting a shell 97 and, the chamber 16 is mounted within the breech block 14 by means of the trunions 54 for rotational movement from a first position (FIG. 3a) in which the opening 94 is aligned with the breech second aperture 66 and the barrel 70 and a second position (FIG. 3b) in which the opening 94 therein is aligned for simultaneously accepting a shell 97 from a point outside the breech block 14 and ejecting fired shell casings 98 therefrom.

In this operation, the chamber 16 is rotated in the direction of Arrow 100 until it hits a stop 102 fixed to a non-moving portion 104 of the gun or surrounding structure, the stop 102 being positioned so that the chamber opening 94 is aligned with the shell 97 which has been moved (by a mechanism not shown) into a shell feeding position.

After the shell 97 has been loaded upwardly into the chamber aperture 94, thereby also causing ejection of the shell casing 98 from the chamber opening 94, the chamber 16 is rotating in a closing direction, indicated by Arrow 106 until a projecting chamber lip portion 108 strikes a breech recess portion 110. Since the chamber 16 is rotated from a fixed closed position to an open position determined by the stop 102, proper chamber opening and closing is assured regardless of gun elevation angle.

As an example, if the barrel 70 is pointing horizontally, that is, zero degrees of elevation, the total rotation of chamber 16 is 90 degrees. However, if the barrel elevation angle is 45 degrees, the rotational movement of the chamber 16 from an open to a closed position is only 45 degrees. Further, since the chamber 16 and the breech/receiver assembly 10 rotate about the same axis when the breech is in a battery position for accepting ammunition into the chamber and thereafter firing it, elevation of the barrel 70 by rotation of the receiver 12 about the trunions 22 does not axially displace the rotating chamber 16 from the stop 102.

That is, despite rotation of the receiver 12, the stop 102 strikes the chamber 16 at the same point, the only change in the rotational movement of the chamber being the amount of rotation from an open to a closed position. This feature enables ammunition feeding apparatus (not shown) to be fixed to the support structure, or assault vehicle, and in an operational relationship with the breech/receiver assembly 10 for feeding ammunition thereinto without any flexible coupling mechanism for accommodating relative movement between the breech/receiver assembly and the feed mechanism due to barrel 70 elevation.

The chamber 16 has spherical surfaces 112, 114 on opposite ends thereof which closely mate with correspondingly curved surfaces 116, 118 of breech block insert plates 120, 122 when the chamber 16 is in a closed position with the opening 94 therein aligned with the breech second aperture 66 and barrel 70 for firing the shell 97.

Upon such firing, longitudinal expansion of the shell 96 forces shell end surfaces 124, 126 to conform to the surfaces 116, 118, respectively, thereby creating a seal therebetween, firing of the shell being accomplished by an electrically operated firing pin or igniter 128 through the plate 120 (FIG. 2).

The combination recoil and counterrecoil means 18 is integrated into the breech on opposite sides of the aperture 44 which receives the chamber 16, by means of first, second, third and fourth longitudinal chambers 134, 136, 138, 140, the first and second longitudinal chambers 134, 136 being disposed on opposite sides of the breech first aperture 46 having axes generally parallel with one another and with the breech block second aperture 66. The third and fourth longitudinal chambers 138 and 140 are also disposed on opposite sides of the breech 14 and adjacent the first and second longitudinal chambers respectively with their axes generally parallel with the first and second longitudinal chamber axes.

Upon examination it can be seen that the combination recoil and counterrecoil buffer means 18 includes two separate recoil and counterrecoil buffer systems, 142, 144 disposed on either side of chamber 16. This symmetrical placement of the two buffer systems about the chamber and barrel eliminates any net twisting moment or torque between the buffer systems 142, 144 and the barrel upon recoil and counterrecoil.

It should be appreciated that hereinafter the discussion of the recoil and counterrecoil buffer means 18 shall be directed to only one of the buffer systems 142 which is shown disposed in the first and third longitudinal cylinders for clarity of presentation.

Generally, the recoil and counterrecoil means 18, shown in an exploded view in FIG. 2, and in various operational positions in FIGS. 4a, 4b, 4c and 4d, includes a fixed piston 148 having a piston head 150 disposed in the first longitudinal chamber 134 and a piston rod 152 which is attached to the receiver 12, or a member attached thereto (not shown), and a floating or movable, piston 156 disposed in the third longitudinal chamber 138.

Hydraulic or other imcompressible fluid is disposed in the first and third longitudinal chambers 134, 138 between a forward face 160 of the piston head 150 and a forward face 162 of the movable piston 156, the first and third longitudinal chambers being in fluid communication with one another through an aperture 166.

An inert gas, such as nitrogen or air, is compressed within the third longitudinal chamber between a rear end 170 thereof and a back face 172 of the movable piston 156 under sufficient pressure to move the movable piston 156 forward after recoil of the breech and force hydraulic fluid from the third longitudinal chamber 138 and into the first longitudinal chamber 134 to return the breech 14 to a battery position as will be hereinafter described in greater detail.

The remaining members of the recoil and counterrecoil means will be more particularly pointed out in conjunction with a description of the operation of the recoil and counterrecoil buffer means 18 as shown in the FIGS. 4a through 4d.

FIG. 4a shows the breech in a battery position with the piston head 150 at a rearward end 174 of the first longitudinal chamber 134. Gas pressure in the third longitudinal chamber 138 behind the movable piston 156 maintains the position of the movable piston against the hydraulic fluid in a forward portion 178 of the third longitudinal chamber 138, and against hydraulic fluid level indicating apparatus 180, which generally includes a chamber 184 disposed within the third longitudinal chamber 138 and in fluid communication therewith, and a piston 186 having an enlarged portion 188 disposed midway along the piston between rear and front piston ends, 190, 192. The rear piston end 190 extends from the chamber 184 into the third longitudinal chamber 138 and the front piston end 192 extends through a receiver wall 198 to the exterior of the receiver 12.

The movable piston 156 is held against the rear piston end 190 by the compressed inert gas therebehind and the piston 186 is held in a rearward position within the chamber 184 by the hydraulic fluid which communicates with the chamber 184 by a port 200. As the inert gas maintains the movable piston 156 against the hydraulic fluid in the third longitudinal chamber 138 and the rear of the piston 186 the level of the hydraulic fluid in front of the piston 156 is indicated by the amount of the front piston end 192 projecting past the receiver end 198, any leakage of hydraulic fluid from the buffer means indicated by an increased projection of the front piston end 192 when the breech is in the battery position.

Turning to FIG. 4b, after firing of a shell, the breech 14 and the barrel 70 connected thereto begin to recoil and move in a rearward direction. During this motion the piston head 150 forces hydraulic fluid from the first longitudinal chamber 134 through the aperture 166 and into the third longitudinal chamber while drawing air into the first longitudinal chamber in back of the piston head 150 through an air port 204. The entry of hydraulic fluid forced by the piston head 150 into the third longitudinal chamber 138 causes the movable piston 156 to move rearwardly thereby further compressing the gas rearward of the movable piston 156, compression of the inert gas absorbing recoil energy and stopping the recoil of the breech 14 and barrel 70 before the movable piston 156 reaches the end 170 of the third longitudinal chamber 138 as shown in FIG. 4c.

After the breech and barrel recoil is stopped, the compressed gas between the rear end 170 of the third longitudinal chamber and the movable piston 156 drives the piston 156 forwardly which in turn forces the hydraulic fluid from the third longitudinal chamber 138 back through the aperture 166 and into the first longitudinal chamber 134 thereby pushing the breech 14 back forwardly in counterrecoil as shown in FIG. 4d.

During recoil, the hydraulic fluid in the first longitudinal chamber 134 is forced through apertures 208, 210 and into the counterrecoil means 212, which generally includes a cylinder 214 mounted within the first longitudinal chamber 134 at a forward end 216 thereof and a tubular piston rod 220 disposed around the piston rod 152 and within the cylinder 214.

Movement of the hydraulic fluid into rearward regions of the cylinder 214 forces the tubular piston 220 forwardly and causes a front end 224 to project in front of a breech forward face 226.

Turning again to FIG. 4d, as the breech 14 moves forwardly during counterrecoil and approaches the battery position, the tubular piston 220 comes in contact with a rear face 230 of the receiver wall and is moved rearwardly. An enlarged ring portion 232 of the tubular piston then forces hydraulic fluid from the cylinder 214 to the rear of the ring portion 232 through the orifice 210 and thereafter through the aperture 208 and back into the first longitudinal chamber.

A sleeve 234 is moved rearwardly as the hydraulic fluid is forced out of the chamber 214 and partially blocks the orifice 208. This restricted hydraulic fluid flow through the orifice 210 rapidly slows the breech and stops breech counter recoil.

Although there has been described hereinabove a specific arrangement of a breech/receiver assembly in accordance with the invention for purposes of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations, or equivalent arrangements which may occur to those skilled in the art should be considered within the scope of the invention as defined in the appended claims.

Claims

1. A breech/receiver assembly for automatic cannon having a barrel comprising;

a receiver having trunion means for pivotally supporting the receiver in a a gun mount,

a breech having means defining a first aperture therein for receiving a chamber and a second aperture therein, communicating with said first aperture and adapted for receiving the barrel at an outside end thereof, said breech being slidably mounted within the receiver;

a chamber having means defining an opening therethrough sized for accepting ammunition, said chamber being mounted within the breech first aperture for rotational movement about a chamber axis from a first position in which the opening therein is aligned with the breech second aperture and barrel and a second position in which the opening therein is disposed at an angle with said breech second aperture and is aligned for simultaneously accepting ammunition from a point outside the breech and ejecting ammunition casings therefrom, said chamber axis being coaxial with said trunion means when the chamber aperture is aligned with the breech first aperture and the breech is in battery position; and,

combination recoil and counterrecoil buffer means disposed within said breech for buffering barrel recoil and counterrecoil forces as the breech slides within the receiver, said combination recoil and counterrecoil buffer means including two fixed pistons and means defining first and second longitudinal chambers in said breech for receiving said fixed pistons, one piston being disposed in each of said first and second longitudinal chambers, said longitudinal chambers being disposed on opposite sides of said breech first aperture and having axes generally parallel with one another and with the breech second aperture longitudinal axis, said fixed pistons being fixed to the receiver and moving in said longitudinal chambers in the breech when the breech moves relative to the receiver in recoil and counterrecoil.

2. The breech/receiver assembly of claim 1 wherein the chamber includes a pair of trunions for mounting the chamber within the breech first aperture, the chamber trunions being coaxially aligned with the receiver trunion means when the breech is in a battery position.

3. The breech assembly of claim 1 wherein the combination recoil and counterrecoil buffer means includes two movable pistons, and means defining a third and fourth longitudinal chamber for receiving said movable pistons, one movable piston being disposed in each of said third and fourth longitudinal chambers, said third and fourth longitudinal chambers being disposed on opposite sides of said breech block first aperture and having axes generally parallel with one another and the breech block second aperture longitudinal axis.

4. The breech/receiver assembly of claim 1 further comprising means for controlling the rotational movement of the chamber to enable the chamber to align with a fixed aperture feeder assembly independent of the elevation angle of the breech/receiver assembly.

5. The breech/receiver assembly of claim 4 wherein the means for controlling the rotational movement of the chamber includes a stop fixed to a nonmoving portion of the automatic cannon.

6. A breech/receiver assembly for automatic cannon having a barrel, comprising;

a receiver having coaxial trunion means for pivotally supporting the receiver in a gun mount;

a breech having means defining a first aperture therein for receiving a chamber and a second aperture therein communicating with said first aperture and adapted for receiving the barrel at an outside end thereof, said breech being slidably mounted within the receiver, said breech having means defining first, second, third and fourth longitudinal chambers herein, said first and second longitudinal chambers being disposed on opposite sides of the breech first aperture and having axis generally parallel with one another and with the breech block second aperture longitudinal axis, said third and fourth longitudinal chamber being disposed on opposite sides of the breech first aperture and having axes generally parallel with the first and second longitudinal chambers;

a chamber having means defining an opening therethrough sized for accepting ammunition, said chamber being mounted within the breech first aperture for rotational movement from a first position in which the opening therein is aligned with the breech second aperture and a second position in which the opening therein is disposed at an angle with said breech second aperture and is aligned for simultaneously accepting ammunition from a point outside the breech and ejecting ammunition casings therefrom; and,

combination recoil and counterrecoil buffer means disposed within said breech for buffering barrel recoil and counterrecoil forces as the breech slides within the receiver, said combination recoil and counterrecoil buffer means including two fixed pistons, one fixed piston being disposed in each of said first and second longitudinal chambers and each having a piston rod attached to the receiver, and two movable pistons, one movable piston being disposed in each of said third and fourth longitudinal chambers, said first and third longitudinal chambers being in fluid communication with one another and the second and fourth longitudinal chambers being in fluid communication with one another and said combination recoil and counterrecoil buffer means including hydraulic fluid disposed in all of the longitudinal chambers, said hydraulic fluid being disposed between a forward face of the fixed pistons and a forward face of the movable pistons, the fluid communication between the longitudinal chambers eneabling hydraulic fluid to flow between the first longitudinal chamber and the third longitudinal chamber and between the second longitudinal chamber and the fourth longitudinal chamber upon recoil of the breech, the fixed pistons moving within the breech first and second longitudinal chambers in response to recoil movement of the breech in the receiver thereby forcing the hydraulic fluid from the first longitudinal chamber into the third longitudinal chamber and from the second longitudinal chamber into the fourth longitudinal chamber.

7. The breech/receiver assembly of claim 6 wherein the combination recoil and counterrecoil buffer means further includes an inert gas compressed within the second and fourth longitudinal chambers between a rear end thereof and a back face of the movable pistons disposed therein, said inert gas being under sufficient pressure to move the movable piston forward after recoil of the breech and force the hydraulic fluid from the third and fourth longitudinal chambers and into the first and second longitudinal chambers respectively, thereby returning the breech to a battery position.

8. The breech/receiver assembly of claim 7 further including monitoring means for determining the amount of hydraulic fluid within the longitudinal chambers.

9. The breech/receiver assembly of claim 8 wherein the monitoring means includes monitor pistons in fluid communication with the second and fourth longitudinal chambers and the hydraulic fluid therein, said monitor pistons having rod means extending through the receiver and having portions observable from outside the receiver for indicating hydraulic fluid level, and rod means being positioned for engagement with the movable pistons when the breech is in the battery position, the positions of the movable piston when the breech is in battery position being determined by the amount of hydraulic fluid in the longitudinal chambers and such position being indicated by the amount of the rod means observable from outside the receiver.