That would have been my guess too. The thread would have taken a plug that could be adjusted in and out so that the precise headspace could be determined. The exact wording of the inscription would help (and will probably prove us both wrong too!)
All still guesses I’m afraid:
10/57 manufacture date
ND Naval dept (if so I think that would make the gun used a 40/60 ?)
ATT FOR INO 61211/2 the instruction number used to tell the Gunfitter how to use etc the gauge.
[quote=“Simon”]Again a guess as I’m not up on the workings of a bofors gun, but I’d think some sort of rod would screw in the end and it could be a headspace guage ?
The style of etching on this gauge is the same as I have seen on the issue small arms gauges when I was an armourer.
We see that something was screwed into the base of this gauge but it looks too complicated to be simply a headspace gauge. The Bofors headspaces on the rim thickness and the only headspace gauge that I have seen was a short steel one mimicking the rear few inches of the case plus rim. There must be more to this expensive looking item than we are seeing at present.
I have had similar items in several calibers and they usually turn out to be chamber gauges. Sometimes they are precise master gauges from which the chamber gauges are made. Sometimes they are gauges for inspecting ammunition manufacturing equipment. Hard to say what the threading means IT MIGHT IMPLY THAT THIS IS A WORK GAUGE INSTEAD OF A MASTER GAUGE. One would need a handle of some sort to pull this out once seated .
I’d guess it’s British, from the Broad Arrow stamps.
I’m also gonna guess it’s a headspace gauge.
FWIW, the US Army field and ordnance maintenance manuals neither show nor mention any tool of similar appearance, description, or presumed purpose. After all, headspace in the 40mm Bofors L/60 is non-adjustable, unless you swap out parts of a different spec. In fact, any headspace-related problems are supposed to be corrected by parts replacement.
So I’m gonna guess further that it’s only for factory use. The only way to increase headspace on a Bofors is to significantly wear the barrel or breech face, or somehow manage to stretch the breech ring(!). Thus, parts replacement cures that problem quickly.
Worth mentioning is also the official lack of any “stuck case removal” tool. Too much headspace could result in a ruptured case, or a piece of a case (neck) stuck in the chamber. As in any automatic cannon, debris in the chamber could really ruin your day…
Again, the barrels on all Allied Bofors L/60’s were totally interchangeable as an assembly (but obviously not between air-cooled and water-cooled guns). One salient feature of the Bofors was the quick-change barrel, so any problems with a worn/hot/damaged barrel were easily taken care of.
Too little effective headspace, as a result of dented/damaged/dirty ammo or debris in the chamber, can really gum up the works in a hurry. Especially if the breechblock almost closes on a sticky round. In this case, the barrel can’t be removed, and immediate action involves clearing the gun, followed by beating the breech block down (open) with a maul…
So despite the fact that headspace issues are just as serious (if not more so) in a Bofors as in any other gun, the causes and cures were so straightforward, with such limited options, that headspace gauges probably weren’t worth issuing to the field or depot.
I think it less likely, but possible, that you’ve got a barrel chamber gauge. It could have been used in a fixture (possibly made of an actual breech ring and block) to check the axial location of the chamber relative to the barrel threads. Even if the chamber was 100% correct in and of itself (as far as the casing profile and the “headspacing” from the rim to the shoulder), if the threads and chamber were not in correct relative location then the actual headspacing of cartridge head to breech face could be off.
10/57 is the month and year of manufacture
INO is Inspectorate of Naval Ordnance
Crowsfoot + ND = Navy Department (oldies will remember Army kit being marked with Crowsfoot + WD for War Dept
The round is a test or inspection round and would need to be manually fed into the chamber which was difficult to access due to the auto loader and hopper. Access to the breech was via a viewing aperture (used to check guns clear) A rod was screwed into the base and fed into the chamber through the aperture, unscrewed and the breech was then closed. A spent live shell case would normally be extracted and ejected using recoil energy but this would not occur when opening the breech manually. The rod would then be reattached to remove the inspection round.
ATT FOR INO 61121/2 This could well be a reference to the rod i.e this is an attachment to be fitted.
[quote=“Simon”]Having asked around I’ve got this to add:
The round is a test or inspection round and would need to be manually fed into the chamber which was difficult to access due to the auto loader and hopper. [/quote]
FWIW I think removing the autoloader (and therefore “hopper”) assembly would greatly facilitate things. Not to mention the single inspection round could be manually dropped through the autoloader, and pushed down onto the loading tray.
Not sure what you mean here? There is a top cover, bottom cover, rear cover, and a side cover. I think I would only trust swinging down the rear cover, to allow clear sight from the back of the gun. With the breech lowered (open), one could see clear out the muzzle (even with the autoloader in place).
Could you post your reference, please, Simon? I’m curious as to what “viewing aperture” they mean. All my sources are US, but everything on my British Bofors is almost identical to the US ones.
Again, I don’t see the necessity of the rod to handle the gauge. It should extract and eject just like a live round or spent casing.
Once a round (or casing) is chambered, lowering the gun’s Hand Operating Lever lowers the breech block. This trips the extractors to extract the shell casing by its rim. There may not be a lot of force there to eject the round or casing rearward, especially since you’ll usually be working to cock the loader’s rammer system at the same time.
However, if you were clever and used something to push the round into the chamber, instead of using the rammer to “throw” the round into the chamber, you could trip the extractors with the extractor trip lever on the side of the gun. This would close the breech while keeping the rammer cocked. That’s why it’s important to clear the gun of all ammo before trying to remove a misfired cartridge…
Either way, standard procedure is to elevate the gun to about 30 degrees, so any hand-extracted round or casing will simply slide out the back by gravity.
Additionally, I find it odd that somebody would screw a rod into the rear of the test cartridge. As soon as it was fully chambered, the extractors would be tripped by the cartridge rim, and the breech block would spring up (trying to close). I think this would be rough on the breech block and the threaded rod! Yes, you could hold back the hand operating lever to keep the breech open, but this implies a two-person job, whereas one person could push a round into the chamber with a broomstick and save the effort.
Perhaps since it was a precision inspection gauge, it was undesirable for it to be roughly handled by the gun’s usual high-speed ramming system?
I’m really curious to see an illustration of this gauge in use.
I asked the question about the gauge on a Brit Army site and the above was one of the answers which most people agreed upon, I myself know nothing about the Bofors, I believe that the question about the markings has been answered correctly and as you say a precision gauge would have been hand “fed”.
I’m trying to get the Naval inspection procedure and standards for the Bofors. Would the Bofors gun in a Naval mount have to be used in a different manner to a ground mounted Bofors ? could this account for an inspection aperture ?