40mm Bofors Question

herr-pear · #1 09-07-05, 05:51

In real life I was an engineer so this is an engineer-type question. If anyone can answer it or point me to a book or other reference that will, I shall be greatly appreciative.

How does the feed mechanism of the 40mm Bofors gun work? Specifically, how does the cartridge get out of the four shot clip and into the chamber? I consider myself a serious student of firearms mechanisms, But haven't been able to find info on this gun. I don't even know if it fires from an open bolt or a closed bolt. I suppose what I really need is a "How this gun works" type of book.

Thanks for any information you can share.

Roger Hull
Las Vegas, NV
Worlds Greatest Impulse Buyer

Godwin Hampton · #2 09-07-05, 11:54

Roger,

The rounds' base rest in the clip depressions, and are retained by a hook which enters a groove in the bottom of the cartridges and locks them in place. The 4 hooks each have a spring-loaded button to release them, much like the lock on a bayonet. These buttons protrude from the clip facing forward, and as the loaded clip is being fed into the autoloader by the feed pawls, the buttons encounter a ramp which forces them open, thus freeing the round. Further down another ramp directs the clip out through a slot, through which it falls at the feet of the loader.

The Bofors fires from an open breech position, as do most automatic weapons. It is dangerous to leave a hot gun loaded!

The rounds are fed down onto a loading tray, underneath the tray is a spring-loaded rammer. When the gun is fired, the rammer is released, projecting the round into the chamber. The cartridge rim pulls the two extractor arms forward, releasing the spring-loaded breech block. This rises and in turn releases the spring-loaded striker which fires the round.

On recoil, the breech block is cammed open, and the extractors throw the cartridge back. This passes over the loading tray on its way out trough the cartridge deflector at the back, eventually being disposed of towards the front of the gun. At the same time the rammer is re-cocked, as is the striker. The feed arm moves up moving the feed pawls over the rounds in the autoloader.

On run-out, the feed arm moves down again forcing a round onto the loading tray in preperation of a repetition of the cycle.

That's the principle, as the mechanism was much more refined with safety gadgets, indicators, etc. It was one hell of a gun!

herr-pear · #3 09-07-05, 14:26

Godwin:

Thank you for your rapid reply. It was very clear and concise, much more so than I had any right to expect. And I agree, it is a hell of a gun.

Roger in Vegas

Godwin Hampton · #4 09-07-05, 16:19

Roger,

Any time! You have touched my favourite subject! It is good to know that there is interest not only in saving and restoring Bofors guns, but also in what made them tick.

I have learned a lot from this Forum, and it was about time I gave something in return. So ask away, someone here will probably know the answer.......

Snowtractor · #5 09-07-05, 18:42

Quote:

Originally posted by Godwin Hampton
Roger,
The Bofors fires from an open breech position, as do most automatic weapons. It is dangerous to leave a hot gun loaded!

It is so obvious that it is easily overlooked. Learn somehting new everyday.
Thx
Sean

Mats · #6 11-07-05, 21:28

A very thorough description from Godwin.

If I may, I would like to add that beside the feed arms with the feed pawls there are also two feed rolls that engages the round for the last step down to the loading tray. They are needed get the cartridge in the exact position for the rammer. The rolls have four very deep longitudinal grooves, giving them a more or less cruciform cross-section.

The feed rolls were omitted on the L/70. But the L/70 also have a deflector tongue to direct the spent case downwards. In this way the new round doesn't have to wait for the empty cartridge-case to be out of the gun-casing before it's fed down to the loading tray.

Godwin Hampton · #7 12-07-05, 00:33

You're right Mats,

The feed rolls (or feed cylinders) also made sure that only one round at a time was fed onto the loading tray. In fact they locked after turning for a quarter of a turn, and only unlocked again on run-out, after the fired cartridge had been ejected. Their function was roughly similiar to a revolving door found in hotels,etc.

The feed pawls, which pushed the rounds down past the feed rolls on run-out, can be seen in the Bofors gun fitted to the Hercules in Stellan's Bofors Gun thread, last page. The pawls just visible in the curved slot just beyond them are the Stop Pawls, which keep the rounds in place while the feed pawls are moving up past them to repeat the feed cycle.

The L70 had a tilting tray, raised at the front during recoil to allow the ejected cartridge to pass out underneath it, to then fall in place, ready loaded, during run-out for ramming.
This was one of the improvements which allowed the rate of fire to actually be doubled from that of the L60.

herr-pear · #8 12-07-05, 03:58

Thank you, gentlemen. These posts have been most enlightening.

I have become aware of the book "The 40mm Bofors Gun" by Terry Gander. It is Out Of Print (OOP) but I was able to locate a copy at a used book seller on the Internet, but at a prohibitive price. Can anyone who has a copy advise me if the book contains a diagram of the firing cycle? I hate to waste the money if it won't answer my questions.

Thanks Again.

Roger in Vegas

Godwin Hampton · #9 12-07-05, 07:18

Roger,

It does not! The content, while very interesting, is more historical than technical.

Some very good diagrams can be found in the 1946 Maintenance Manual, which every now and then may appear on e-Bay, but the prices of anything Bofors are going through the ceiling.

As soon as I learn how to scan and post pictures I will do so, as there seems to be some interest.

Stellan Bojerud (RIP) · #10 12-07-05, 09:51

Terry Gander´s book on the 40 mm Bofors gun is interesting but filled with errors.

BTW the idea to the mechaniasm of the 40 mm Bofors L/60 has its orign in the 57 mm M/1889 naval gun.

Mats · #11 12-07-05, 23:32

Bofors 40 mm L/60 ready to fire
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Just discharged
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Recoiling, breech opening
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Going into battery, new round just about being rammed
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Breech closing, just about to discharge again
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Godwin Hampton · #12 12-07-05, 23:52

Mats,

Lovely diagrams! Just what Roger wanted,I suppose.

Are they from a Bofors technical publication, and is it available?

Thanks for sharing them.

Mats · #13 13-07-05, 00:08

It's from the gun's manual.
How I wish I know this five years ago - I could have bought a dozen of these manuals for 3 euros a piece - now I only have one.

Godwin Hampton · #14 13-07-05, 00:12

Well Mats,

If you come across one again, let me know!

herr-pear · #15 13-07-05, 00:22

WONDERFUL! Please, what is the source of these diagrams and where can I get a copy?

My most humble thanks. I love this group. You guys are the best!

Roger in Vegas

Mats · #16 13-07-05, 00:39

You're welcome.

It's from the official hand book.
"Beskrivning över 40 mm Luftvärnsautomatkanon m/36" 1943
(Description of 40 mm Anti-aircraft automatic cannon model 1936). It's about 250 pages with 15 fold-out drawings.

If I happens to come across a copy or two I'll make sure to let you both know - but there's no promises.

Rob Fast · #17 14-07-05, 02:16

feed prawls and loading tray

Breech open and extractors, rusty and I am not happy about that.

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Single shots loadied thru this back shell discharge opening

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ok

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herr-pear · #18 14-07-05, 05:40

This forum is a treasure trove of information. I can not think of how to thank you enough.

Roger in Vegas

Godwin Hampton · #19 14-07-05, 07:36

Hi guys,

Rob, I was going to ask you if you could (again) post some pictures to show the mechanism, and here I find them! You must be a clairvoyant or something!

Roger, NOW you can understand, with Mats' diagrams and Rob's photos. A picture is worth a thousand words, so they have provided a whole book!

Still, if anything is still not clear, this is the place to ask.....

herr-pear · #20 14-07-05, 09:08

Correct me if I'm wrong, but the photo of the single shot loading opening is upside-down, is it not?

(See, I really am paying attention

).

Roger in Vegas.

Mats · #21 14-07-05, 12:05

It's the correct orientation.

The big bright part is the loading tray.
The big nut is part D23 on the first diagram.

The two latches, one to the left and one to the right, is the parts of the rammer that engages the rim of the cartridge.

You can see the edge of the RH feed roll as a bright arc above the RH latch. (On the first photo, you can clearly see the two feed rolls.)

Godwin Hampton · #22 14-07-05, 13:49

May I add something, Mats,

As it is now, the first round to be fed by the loader pushing two clips down hard into the autoloader will have the cartridge rim fall squarely into the two rammer levers. When the gun is fired, the rammer is released and flies forward. The levers move in grooves which open outwards at the end of travel, making the levers release the round to continue on its way into the chamber.

The rammer has to be rather powerful to flick the rounds into the chamber, especially at high angles of elevation.

As it is in the photo, rounds cannot be loaded ( or unloaded ) as they will not clear the rammer levers. First the Hand Operating Lever on the outside of the gun has to be moved right back all the way, over-cocking the rammer. The grooves also open out towards the rear, opening out the levers and allowing the gun to be loaded/unloaded.

This was convenient if for example an armoured car had to be engaged. The AA round on the tray would be removed, and replaced with an AP round without disturbing the rest of the ammunition in the autoloader. The round on the tray could also be removed and the rammer released during periods of inactivity, to render the gun safe and to 'ease springs'. By moving the Hand Operating Lever to the rear to re-cock the rammer, and pressing down on the rounds again the gun would be loaded.

Mats · #23 14-07-05, 15:35

As Godwin pointed out the ramming is rather violent, and here's a picture of what can happen if the cartridge case rim give way.

Godwin Hampton · #24 14-07-05, 17:04

Roger,

If you want to read more about weapons, visit Mats' homepage!
FANTASTIC!

Look it up on his profile page.

Rob Fast · #25 15-07-05, 02:37

from a Bofors owner trying to understand how this unbelievable piece of engineering came about and how it all seems to work so beautifuly. Cheers Rob

herr-pear · #26 15-07-05, 03:44

Mats: I just visited your homepage and it is just fantastic. Bravo!

One small question left on the feed mechanism; After the cartridge is released from the four round clip, how is it moved forward onto the feed rollers? I think this is only a very short distance but surely it must be done to prevent the base of the cartridge from hanging up as the feed rollers turn. OR, I could be completely wrong and have overlooked something. I can think of three possible methods of doing this; but Bofors probably did it a fourth way.....

Many, many thanks again.

Roger in Vegas

Alex Blair (RIP) · #27 15-07-05, 13:17

Quote:

Originally posted by Rob Fast
Breech open and extractors, rusty and I am not happy about that.

Rob...
Put the G-96 to the old girl and smoke a few rounds through her at the neighbours...That will tune her up...

herr-pear · #28 18-07-05, 06:42

Please disregard the last silly question. By studying Mats diagrams and Robs photos and banging my head on the desk it became clear. I'd give a pretty to see one of these in the flesh.

BTW, I just won a book on Ebay that should be helpful also. It is titled "War Machine #88, WW2 light AA Guns" with emphasis on the Bofors. Should be in my hands within a week or so.

Thanks again, everybody

Roger in Vegas

Godwin Hampton · #29 18-07-05, 07:24

Roger,

Good luck with the new book, but they usually lead nowhere!

When loading the Bofors, the Loader pulled the large 'Hand Operating Lever' backwards as far as it goes and places it in the rear catch. This keeps it in a vertical position, with the rammer now cocked,the breechblock open (down) and the striker cocked.

4 (the loader in British gun drill) drops one charger (clip) into the autoloader and presses down heavily on the top round until the bottom one drops onto the loading tray ( he could check through the rear aperture if in doubt ). He then inserts a second charger in the autoloader and unhooks the hand operating lever and places it back in the front catch.

All he has to do then is to move the 'Foot Pedal Held' lever ( what you and I would call the Fire Control Lever) from FPH to SINGLE or AUTO as ordered, and back on 'Cease Firing'. He also had to keep up with the gun's appetite, dropping one charger every two seconds when firing on Auto.

Hope this helps clarify the picture.

herr-pear · #30 18-07-05, 18:32

Godwin:

Again, my deepest thanks for your help. I sent the check for the book this morning and will report on it when it arrives. The cost was only $7.00 (USD) so I can't really lose.

I find it inspiring that a man in the USA can ask a question and get answers from Malta, Sweden, and Canada almost instantly. Bless you all.

Roger in Vegas