Weapon calibre sizes

[roadiemullet]

Ok this stems off from a discussion with another forum member-Jonny(FGM)- that we were having the other day as I was annihilating him on our latest cmsf game. We were talking about gun calibre sizes (a nerds discussion if ever there was one) and the weird numbers that various armies have come up with over the years. Some sizes seem simple enough, 7.62mm for instance is 1/10th of the 76.2mm or 3" tank gun turret. Others are nicely rounded up, such as 30mm, 40mm etc.

But then you have weird sizes like the British 81mm mortar. Why 81mm?? Why not 80 or 85? Why 51mm? why not nice and simple like the 60mm mortar? Why is the LAW 66mm and not 65?

I had a theory that the Germans chose sizes like the 37mm and the 88mm because they are (roughly rounded down and up) halfway between 25-50 (37.5) and 75-100 (87.5). Is this correct? If not why did they choose such seemingly random but precise numbers?

Google has produced dire results so I thought Id ask here as there are people much more knowledgeable about this sort of thing in this forum than in most others...

[ZPB II]

I think the 81mm thing had something to do with being able to use the opponent's 80mm ammunition but he couldn't use the 81mm one...Dunno if anyone used 80mm mortars though.

[JonS]

7.62mm for instance is 1/10th of the 76.2mm or 3" tank gun turret.

I'd be astonished if that were the reason.

But then you have weird sizes like the British 81mm mortar. Why 81mm?? Why not 80 or 85? Why 51mm? why not nice and simple like the 60mm mortar?

3-in and 2-in respectively.

I had a theory that the Germans chose sizes like the 37mm and the 88mm because they are (roughly rounded down and up) halfway between 25-50 (37.5) and 75-100 (87.5).

I think you need to work on your theory a bit

You probably want to look at throw-weight and range and MV and propellant energy density and lathe sizes and a host of other factors. Designing anything is a tradeoff betweeen wants and needs and cost and capability. It's seldom as simple as something like 'let's just pick something halfway between x and y'

Also, bear in mind that some stated calibres are not the actual calibre of the weapon concerned. I know it sounds odd, but frankly soldiers don't care too mcuh what the exact calibre of their weapn is - they just want to get the right ammo. For at least one WWII tank cannon the calibre was deliberately mis-represented so that ammn for it would not be confused with another gun of a similar barrel calibre but with incompatible breech cavity.

[JonS]

I think the 81mm thing had something to do with being able to use the opponent's 80mm ammunition but he couldn't use the 81mm one...Dunno if anyone used 80mm mortars though.

There was an urban legend that the Russians (the Soviets) supposedly designed their mortars at 82mm so they'd be able - in an emergency - to fire any captured rounds since everyone else used 81mm, while they in turn would not be able to fire the Russian ammn since it was 1mm oversized.

I've never found that to be particularly credible.

[Michael Emrys]

3-in and 2-in respectively.

Er, not exactly. As you well know, 3 inches is 76.2 mm...more or less (these things almost never work out to the nth decimal place). You are right on (again more or less) about the 2" mortar being 51 mm though.

About the larger question, 88 mm was the bore of a German naval cannon at the turn of the 20th. century. I expect it would be a pretty safe bet that many of the bore sizes that made it into the middle of the century—and in some cases well beyond—began their lives as black powder weapons firing a cannon ball of that size which happened to weigh some nice round number of pounds or whatever the local measure was.

Michael

[AslakH]

The German 77mm cannon (FK96n/A) got its calibre from the idea that once captured, the enemy could not use it with minor alterations, since the closest French gun was 75mm.

Us that make furniture say that it's easier to take something off, than to put something on. You can't make a 77mm barrel into a 75mm barrel. But the 75mm could be made into the 77 rather easily. Kinda nifty.

[Michael Emrys]

You can't make a 77mm barrel into a 75mm barrel.

I'm not so sure. Seems to me that relining the barrel would be a relatively simple matter, although I admit that I haven't researched the matter deeply.

Michael

[AslakH]

Relining would cost more, and take more resources AFAIK.

Drilling it up to 77mm would take less time and money.

I don't know what kind of facilities the French had during the Great War, so I can't be 100% sure. I also can't comment on the tolerances regarding the barrels (thickness, etc) that may have contributed to not modifying the captured guns.

Perhaps it was mere pre-war "good ideas".

[Michael Emrys]

Relining would cost more, and take more resources AFAIK.

Drilling it up to 77mm would take less time and money.

So? Relining a captured piece would still cost less than manufacturing an entire new piece. I recall that during WW II the Germans rechambered captured Soviet 76 mm to accept their 75 mm round. I am inclined to believe that rechambering is a more expensive process than relining the barrel, which for many larger pieces is routinely done anyway.

Michael

[flamingknives]

Of course, there is always the question of where are you measuring to? Lands? Base of rifling?

[Michael Emrys]

Life is so complicated!

Michael

[AslakH]

Well, I was talking about pre-1914 French.

Don't know how it worked out in practise.

Sometimes you get a really good idea, but it turns out it's not all that great.

Might be the case with that gun?

[JonS]

Many pieces of ordnance used a liner inside the abrrel which 'carried' the rifling. Once the rifling was worn you could extract the liner and replace it, without having to replace the whole barrel.

The British re-lined their leftover 8-in WWI hows to become the WWII 7.2-in how. Replacing the liner for a 77mm weapon with one suitable for 75mm would be a fairly straightforward task.

Besides, choosing your calibre based on what would be least useful to the enemy seems like a bizarre design criteria. Why not chose something that'd be most useful to you, instead?

[Praetori]

Ok this stems off from a discussion with another forum member-Jonny(FGM)- that we were having the other day as I was annihilating him on our latest cmsf game. We were talking about gun calibre sizes (a nerds discussion if ever there was one) and the weird numbers that various armies have come up with over the years. Some sizes seem simple enough, 7.62mm for instance is 1/10th of the 76.2mm or 3" tank gun turret. Others are nicely rounded up, such as 30mm, 40mm etc.

But then you have weird sizes like the British 81mm mortar. Why 81mm?? Why not 80 or 85? Why 51mm? why not nice and simple like the 60mm mortar? Why is the LAW 66mm and not 65?

I had a theory that the Germans chose sizes like the 37mm and the 88mm because they are (roughly rounded down and up) halfway between 25-50 (37.5) and 75-100 (87.5). Is this correct? If not why did they choose such seemingly random but precise numbers?

Google has produced dire results so I thought Id ask here as there are people much more knowledgeable about this sort of thing in this forum than in most others...

Nice and round numbers mean little in production.

A lot of factories or more importantly, machinery tools and molds for tools have been developed during the ages. A lot of weapons are designed to fire a specific projectile and not the other way round.

There are projectiles of nearly every caliber that's already been built. Patents and known characteristics of course also affect why certain models are brought forward for large scale military use instead of others.

[Erwin]

Good point Praetori. Likewise, the WW2 Tiger was built around the 88mm flak gun.

[dan/california]

There is a relatively coherent theory floating around out there to the effect that the standard railroad gauge goes back Roman chariots. There can be deep and non obvious reasons for some of these things.

[Michael Emrys]

Like I suggested, check cannonball sizes.

Michael

[roadiemullet]

There is a relatively coherent theory floating around out there to the effect that the standard railroad gauge goes back Roman chariots. There can be deep and non obvious reasons for some of these things.

dayum, I guess I never thought about it that deep. I knew that calibre sizes were more likely to do with more older (pre 1900) gun sizes which were measured in weight among other things its just that the 37 and 88 seemed too much of a coincidence. Im sort of basing this on that ive seen the 21.4mm panzerabwherthingy referred to as the 21mm and 22mm and just assumed perhaps that the same was being done with the 37 and 88.

Having said that, modern stuff like the law isnt based off older cannon sizes as far as I know so why again is it "irregular" sizes like 66mm? I mean thats just an example. Other stuff like the 120mm on the Challenger 2 seems fairly neat enough. Surely the mere 1 or 2 mm of difference cant make THAT much difference to the performance of a projectile? Its just it seems odd to me that if it does and these things are based on factors other than their calibre size in terms of performance (resulting in odd numbers), how come they are then referred to them in that manner?

Btw thanks for the response, we've (me and JonnyFGM) have been pondering on this for a while, so kudos to you chaps for your info

[Lethaface]

Sometimes we tried to find logic explanations for stuff that has become reality with a lot of coincidence influencing it's existence...

[Michael Emrys]

Sometimes we tried to find logic[al] explanations for stuff that has become reality with a lot of coincidence influencing it's existence...

That's worth keeping in the forefront of one's mind.

Michael

[Kanonier Reichmann]

It seems like the Germans were ahead of the game yet again with their calibre sizes. The British 25 pounder fired 87.5mm sized shells and yet there was the 88mm Flak gun. Just slightly larger again to allow the use of the enemies shells but not vice versa. Coincidence?

BTW, I do know the answer is certainly yes. Sheer bloody coincidence!

Regards

KR