I’ve just been looking through my documents relating to 7th Loyals / 92nd LAA 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).
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. Simpools Rob
And all of the above was done at the rate of 120 per minute Unreal And presumably my Dad had to know all about the truck that towed it as well! regards Robert
tmac Many, many thanks for your efforts in transcribing this excellent document. For my sins, although I was in Light Ack Ack for quite some time (Dec 1942 to Dec 1944) I never knew exactly how the Bofor was fired, all I knew was that it was safer for my wireless truck to be sited as far as possible from the bloody gun as possible !!!!!!! Best regards Ron
Thought you might like to see a Bofors live firing with sound. British Pathe - MACHINE GUN FIRES SHELLS
Thought you might like to see a Bofors live firing with sound. British Pathe - MACHINE GUN FIRES SHELLS Was it normal practice to have a Machine Gun mounted on top of the main barrel ?
thanks Owen great footage interesting to see the machine gun (assume a Bren) wondered if it was used for sighting ? regards Clive
Certainly looks like a Bren, no idea why it's there tho'. The sights were much different by 1944 and they were connected to the 'Stiffkey Stick' which allowed the 'aimer' to offset depending on his estimated speed of the enemy plane. The gunners in F Troop 92nd LAA Regt never used it in action and look at their success. By the way when at the practice firing camp at Cartmel, to protect the pilot towing the drogue the chap who fired the gun with the foot pedal, had a rope round his ankle with an officer at the other end. We all had to have a go, including the wireless operators.
a take from the american set up Antiaircraft Artillery - Bofors 40mm Antiaircraft Gun <TABLE style="BORDER-COLLAPSE: collapse" border=0 cellSpacing=0 borderColor=#111111 cellPadding=0 width="98%"><TBODY><TR><TD width="100%" align=middle> </TD></TR><TR><TD width="100%">Equipment of the 40mm fire unit: Bofors gun, director, director chest, cabling, generator and .50 caliber machine gun on M3 mount. Throughout the war, 40mm fire units replaced the single water-cooled machine gun with the quad mount M51 trailer. Antiaircraft units that abandoned the use of the M5 director simplified their setup by eliminating all of the above, except the guns. THE ROPE ATTACHED TO THE FOOT </TD></TR></TBODY></TABLE>
There is a 40mm Bofors on a plinth in Swansea by the river. Its an interesting looking piece of kit! Nice video find Owen.
Regarding the rope attached to the foot of the loader/firer on a Bofors gun. Here is some info I gave earlier to another forum member: The order on the gun for a seven-man Bofors crew was: 1. Gun commander (Sergeant) 2. Layer for elevation (Bombardier) 3. Layer for line (Lance Bombardier) 4. Loader / firer (Gunner / Driver) 5. Ammunition supply (Gunner / Driver) 6. Ammunition supply (Gunner) 7. Target spotter (Gunner) However, I don’t think that order was strictly adhered to, especially when it came to ranks. My father, whose rank was Gunner in F Troop, 92nd LAA, was layer for line on his Bofors and the loader / firer also had the rank of Gunner. Layer for elevation was the man who raised and lowered the barrel by means of a hand crank. Layer for line was the man who traversed the barrel from right to left, or left to right. Both these men were seated either side of the gun, just forward of the breech mechanism. The gun was fired by means of a foot-operated trigger, which was pressed by the loader / firer on orders from the gun commander, the sergeant. The two ‘ammo numbers’ – No 5 and No 6 – continually filled empty clips (known as ‘chargers’) with fresh shells and handed them to the loader / firer, who placed them into the breech. Being an ammo number was a strenuous job, and they were often hard put to keep up with supplying clips of shells because of the gun’s high rate of fire. My father’s old comrade recalled his job as loader / firer: ‘The sergeant would shout, ‘Enemy aircraft!’ and the gunlayers would track the plane, elevating and traversing the gun barrel. When they had it sighted, No 2 would say ‘On!’ and No 3 would say ‘On!’ and the sergeant would look at the direction of the gun and he’d say, ‘Fire!’ and I’d fire. ‘It fired on automatic. You could have single shots, but if you kept your foot down, it fired on automatic – 120 shells a minute. It was noisy and after firing for 20 seconds or so, it became a bit misty. I don’t know how No2 and No3 could see the plane once we’d started firing, because the cordite fumes used to come out of the end of the barrel and make it difficult for them to see. ‘There’d be a rope attached to my foot and the sergeant would be behind. He’d be looking at the sky or looking around at whatever we were firing at. And if he wanted to stop, he’d shout, ‘Stop!’ and pull the rope. So he’d pull your leg off the firing mechanism and you couldn’t fire any more. ‘As soon as he pulled that, I’d bend down and put the gun into Safety and I had to shout, ‘Held!’ to tell everybody the gun was safe.’ The gun crews were supposed to use the aiming apparatus – the ‘Stiffkey Stick’ – to track enemy planes. But I think that in many cases, especially in emergencies, they just fired over open sights.
That is really impressive footage of the Bofors firing - thank you for posting it, Owen. I think Jim, you're correct about the purpose of the Bren gun - it's probably to assist aiming. The idea is to use the Bren with tracer rounds to zero in on the target, and then to fire the 40mm rounds. The idea, I suppose, is to save effort in loading shell after shell, and to conserve ammunition. But the technique would presumably only be used in training and not on the battlefield. I believe modern tank crews also use machine guns to zero in on targets when they're training, because their ammunition costs a fortune and can't be wasted.
