Wartime notes on operation of the 40mm Bofors Gun

I’ve been looking through my documents relating to my late father's wartime unit, 92nd (Loyals) Light Anti-Aircraft Regiment, Royal Artillery, and found these notes I was given about ten years ago on the operation of the 40mm Bofors Gun.
They were compiled by the late Lieutenant (later Captain) John ‘Jack’ Prior during his time with the regiment, probably during training in Scotland in the run-up to D-Day.
I transcribed them from Jack’s original handwritten notes, which were incredibly hard to read, so there are some missing words and there were words which I couldn’t decipher. Also, there may be bits of the document missing and the whole thing seems to be incomplete.
It’s all so highly technical that I can’t understand any of it. However, I thought the notes may be useful to anyone with an interest in the Bofors and may even be helpful to anyone who owns one (I know you can still buy them) or is restoring one.
Also, it shows there was a lot of stuff to be learned by an artilleryman during the Second World War – it wasn’t just a question of ramming in a shell and firing the gun.
These are the notes …

SEQUENCE OF ACTION OF GUN
Action on recoil:
The loading tray and rammer carried to rear, firing mechanism cocked. Breech opened, empty case ejected through the open rammer claws. Breech retained open by the extractor feed pawls rise. Rammer rides over centre check lever.
Action on run out:
Rammer held to rear by centre check lever. Loading tray taken forward, allowing rammer claws to close. Rammer spring compressed. Feed cylinder catches are released by the spring pawls on the loading tray. Feed pawls pulled down, forcing one round through the cylinders on to the loading tray. The rim of the cartridge case in the claws of the rammer. Feed cylinders relocked by catches.
At full runout, centre check lever is depressed by the boss on the loading tray. Rammer spring takes rammer with round forward. Rammer claws opened by the grooves in the loading tray, rammer checked and round thrown forward into the bore. Rim of round striking extractors, takes extractors forward, breech released. Breech closed by breech spring, firing pin released. Gun fires.
AUTOMATIC LOADER
Base, frames, rear guide, front guide, crosspiece, loading tray, rammer, rod, head, spring, spring seat, shoe, levers, catch check-release devices, feeding device, feed cylinder catches, feed cylinders, shafts, plungers. Catch release pistons, rammer cocking lever shaft, arm, cocking lever.
ACTION OF THE AUTOLOADER
1. Automatic loading.
(a) On firing, the barrel recoils and takes with it the breech ring and loading tray carrying the rammer.
I. Breech opened, cartridge case ejected
II. Projection on underside of loading tray releases rammer lever, permitting rammer catch to be raised by spring.
III. Loading tray pawls ride under long arms formed on cyl. catch heads and are reasserted under action of springs in rear of arms.
IV. Rammer shoes rides over the catch and release levers which are resserted under action of springs in front of rammer shoe.
V. Rollers on lower ends of two feed rods ride in eccentric grooves of loading tray. Grooves raise rollers and feed rods and pawls ride over cartridges in the feed guide, being reasserted by action of springs as soon as cartridge passed.
(b) On run-out, barrel takes with it breech ring and loading tray.
I. Rammer head temporarily held by catch lever.
II. Loading tray pawls engage long arms formed on cylinder catch heads, heads rotated so that short arms are clear of the upper sides of cylinders and spring-loaded pawl in each head engages under the side of its respective feed cylinder.
III. Rollers on feed rods force rods downwards and feed pawls force cartridges in feed guides over the stop pawls. Lowest cartridge, forcing feed cylinders apart and rotating them a quarter-circle, is loaded on to tray. Feed cylinder spring-operated plungers return cylinders to receiving position. Spring-loaded pawls in catch cylnder heads are raised to permit cylinders to (blank).

SAFETY ARRANGEMENTS
1. The breech block cannot be closed when the barrel is dismounted because the safety plunger in the breech ring is forced to the front by the action of its spring and, in this position, the flange on the plunger engages in the recess in the block and reains it in the open position (note: the block must be open and the extractors rotated to clear the vertical recesses in the barrel before the barrel can be rotated in the breech ring).
2. The top cover is secured in the open position by the locking catch, ensuring that while dismantling and assembling a barrel, the control arm on the barrel catch engages the breech ring stop on the left side breech casing and prevents the breech ring, etc, slipping back into the casing.
3. The breech block locking bolt, when inserted though the left side of the breech ring with the breech open, prevents the breech block being inadvertently closed.
4. Foot pedal held (and) fire lever prevents the gun being fired by the foot pedal when set to FOOT PEDAL HELD. It also prevents the gun being fired automatically when set to SINGLE FIRE.
5. The feed rods and spring-loaded plungers normally force the feed pawl holders down, but should a jam in the feed device occur, the springs permit the feed rods to operate independently of the holders and prevent damage to the other parts.
BREECH MECHANISM
Breech block, crankshaft, closing spring, right inner crank, left inner crank, check plunger, outer cocking lever, inner cocking lever, outer crank, hand operating lever rod, l-o-l and guide bracket and crank, l-o-l shaft, l-o-l bearing brackets, hand operating lever and shaft arm, catch brackets, extractors, extractor spindle, e-s arm, ex-inner releasing lever, ex-and-l spring, ex-and-l anchor stud, ex-outer and l striker and spring, striker spring cover.
a) To close the breech
Held open by extractors, closing spring tensed, striker cocked by inner cocking lever. Hand op lever to rear. Press forward ext. outer release lever, rotating inner lever and tensing spring, inner lever rotates ext. spindle arm and extractor spindle and thereby extractors, (firing hooks) from breech block. Block now raised by closing spring, under ontrol by op. lever.
Closing spring rotates cover and thereby crankshaft splined to it, and left and right inner cranks, lugs riding in horizontal grooves in either side of block raise it to closed position. Lugs continue to move along inclines until they support block and it cannot rotate crankshaft through inner cranks. During rotation, left inner crank moves away from outer cocking lever (stationary) and bevel is moved from left end of check plunger: Bevel projection on right inner crank, when closed, engages bevelled end of check plunger and forces it to left, compressing spring and bringing curved slot opposite lower arm of inner cocking lever. Check plunger moved from cock notch in inner cocking lever, which can rotate under action of striker main spring. Outer crank rotated by crankshaft to breech closed position.
Striker main spring, released by inner cocking lever and check plunger, carries striker violently forward on to cap of cartridge and fires it.
The outer releasing lever is released immediately the block closes and the inner and outer releasing levers are returned to normal position by springs.
In auto, round engages extractors and releases breech block …
b) To open the breech
Hand op lever pulled to rear, rotating L-op-lever shaft, pulling l-op-l rod down and rear, through guide bracket, movement of rod being controlled by l-op-l and crank pivoting in breech casing. Projection on l-o-l engages lower arm on outer crank and rotates crank anti-clockwise. Outer crank rotates crank shaft, and thereby right and left inner cranks, and tenses closing spring by rotating spring casing.
At first, block not moved, but bearing surface on left inner crank bears on outer cocking lever, which is rotated and with it inner cocking lever, which bears on shouder of striker and withdraws it to rear, compressing spring. At same time, right inner crank bevel leaves the bevel on check plunger held by lower arm of inner cocking lever, engaging the curved slot of plunger.
Further rotation of inner cranks brings lugs into horizontal position in grooves of breech block, which is forced down into fully open position and closing spring cover rotates and tenses spring. As breech block descends, flanges on ribs engage (tops) of extractors, which are rotated, and case ejected. At same time, left inner crank forces down outer cocking lever and inner cocking lever until cock notch is clear of recess in check plunger. Check plunger forced to right by its spring assisted by bevel on left inner rank engaging left end of plunger, to engage cock notch and retain inner lever and striker in cocked position. When pressure on l-o-lever released and lever folded forward, closing spring rotates crankshaft and inner cranks, raising block until upper ends of ribs engaged by hook-projections on extractors, being retained in loading positions.
Rotation of l-op-lev shaft rotates arm on inner end. Arm operates rammer cocking levers (see autoloader). H-o-l folded forward on completion and with it l-o-l rod, releasing outer crank.
In auto, gun recoils and and roller in outer crank runs along cam in op cover in left side breech casing, rotating outer crank … (extractors hook on runout).
Lever forward during autoloading and firing.
… rotate quarter-circle and then limit rotary movement of feed cylinders.
IV. Loading tray pawls, after (rotating?) catch cylinder heads, ride by long arms formed on catch cylinder heads, which are rotated to normal positions under action of torsional springs. Heads withdraw pawls from feed cylinders, pawls being returned to normal positions by springs and short arm of each head engages upper side of its cylinder, preventing rotation.
V. Projection on loading tray engages rammer releasing lever, which pivots and rotates catch lever clear of the rammer shoe and compresses catch lever spring.
VI. When catch lever withdrawn, rammer shoe carried forward by spring to be engaged by feed control check lever, or firing gear check lever if operative. If neither operative, rammer shoe carries rammer levers and cartridge to front until rammer head engages rammer buffer, cam guides in tray open rammer levers and cartridge released to travel into breech under inertia.
VII. Breech closed (extractors released).

 

Thank you very much. I am writing about my father who was with a Canadian LAA - the 3LAA - and having information on the 40mm Bofors is greatly appreciated!

 

Thanks for your message. I've written a history of my late father's LAA regiment, which served in North West Europe during the Second World War. You may find it of interest, especially in relation to your research into your father's regiment. The history is called 7th Loyals / 92nd (Loyals) LAA Regiment RA, 1940-1946 and I've posted it in the Unit History section of the World War 2 Talk website at World War 2 Talk
If you have any trouble finding it, I can e-mail you a copy direct.

 

Thanks, tmac. I'm a member at WW2Talk as well so shall have a look there. Found Ron's thread there re LAA unit.