why is it come in 81mm/82mm instead of 80mm?

[Snow Leopard]

Out of curious...I wonder why Mortar's cal is come in either 81mm or 82mm instead of 80mm?

[Corvidae]

I would suppose that its because those are the calibres that were made,,,,

[John D Salt]

Originally posted by Snow Leopard:

Out of curious...I wonder why Mortar's cal is come in either 81mm or 82mm instead of 80mm?

Because that was one of the bore diameters chosen by Wilfred (later Sir Wilfred) Scott-Stokes when he invented the modern mortar.

http://www.firstworldwar.com/bio/stokes.htm

Stokes mortars came in two calibres, nominally 3-inch and 4-inch. Originally, they fired cylindrical projectiles of those calibres, which were seated in the bore by two rings that fitted the bore diameter, about a fifth of an inch wider -- thus the rings were the only parts of the projectile that needed to be manufactured with precision. Of course, this means that the bore diameters of the 3-inch and 4-inch Stokes mortars were 3.2 inches and 4.2 inches respectively.

Later, the French Brandt design of finned bomb was adopted to give the Stokes-Brandt pattern of mortar that persists to this day. This also explains why the British called their standard WW2 mortar a 3-inch mortar when it was really an 81mm, like everyone else's. Weapons nominally described as 3-inch, 8cm, 81mm, 82mm, 4.2-inch, 10cm, 105mm, 106mm and 107mm mortars all perpetuate the original Stokes calibres, and the word "mortar" in modern usage now almost invariably means a Stokes-Brandt pattern mortar.

All the best,

John.

[General Bolt]

I once heard the Russian used 82 vice the german 81 because 81 bullets would fit in a 82 tube but 82 bullets would get stuck in 81 tubes. We can capture their bullets but they can't capture ours. Same reason for 75mm and 76mm AT guns. I never really read any research to back this up.

[John D Salt]

Originally posted by General Bolt:

I once heard the Russian used 82 vice the german 81 because 81 bullets would fit in a 82 tube but 82 bullets would get stuck in 81 tubes. We can capture their bullets but they can't capture ours. Same reason for 75mm and 76mm AT guns. I never really read any research to back this up.

Jane's Book of Infantry Weapons for 1975 shows on pp. 707-708 drawings of three Soviet 82mm mortar bombs. The diameter over the bourrelets is given as:

bomb________diameter

O-832_______3.202 inches

O-832D______3.216 inches

O-832DU_____3.2 inches

The same source gives the internal diameter of the British 3-inch mortar as 3.208 inches. Given the sorts of tolerances I imagine these things to be manufactured to, I think they are effectively all the same size.

Of course, there's no way of really telling, short of getting an 81mm mortar tube and stuffing 82mm ammunition down it. But on page 726, there is a drawing of a Yugoslav mortar bomb, described as "The 81mm HE bomb. (The Yugoslavian 82mm HE projectile, Model M31)". The drawing shows it with a diameter over the bourrelets of 3.210 inches, and the number "82" written on it. It is fired from the 81mm mortars M31 and M68, respectively copies of the US 81mm M1 and French 81mm Brandt MO-81-61-L.

All the best,

John.

[Mordred]

actually thats a big difference.

I myself do not know what clearance is required between bore and round, but .016" on the plus would be more than enough to create a problem.

and btw 832 is 81.28mm

832D is 81.69mm

take it from a machinist, that's a notable differnce

[John D Salt]

Originally posted by Mordred:

actually thats a big difference.

I myself do not know what clearance is required between bore and round, but .016" on the plus would be more than enough to create a problem.

and btw 832 is 81.28mm

832D is 81.69mm

take it from a machinist, that's a notable differnce

But this "notable difference" is between one "82mm" bomb and another "82mm" bomb. Both are fired from the same tube.

All the best,

John.

[Zalgiris 1410]

Hmmm, I am particularly partial to on board mortars, 60-81/82mm types, though I would love 4.2 inch if CM had them on the board.

I have read a fair few times of accounts of British or Commonwealth troops on the one hand complaining that their 3" mortars were out ranged by German and even Italian medium mortars which is not how it is in CMAK of course and on the other is that when they captured German 81mm mortar rounds they had to borrow from the Americans their 81mm mortars to fire them because the rounds did not fit in their own 3" medium mortars at all.

[John D Salt]

Originally posted by Zalgiris 1410:

[snips]

I have read a fair few times of accounts of British or Commonwealth troops on the one hand complaining that their 3" mortars were out ranged by German and even Italian medium mortars which is not how it is in CMAK of course

The range of an infantry mortar depends largely on how many secondaries (or "augmenting charges", US "increments") you can safely stuff down the barrel to send the bomb on its way. The Mk 4 barrel was designed to take a heavier charge than the previous marks of barrel, and so could take a bigger charge for longer range.

PRO document WO 291/491, "Notes on R & A trials of British 3-in and German 8cm mortars", published in 1943, notes that between these two mortars "It is safe to conclude that the difference in maximum range using top service charge is small." It also reports one shoot on which a 3-in mortar was fired using 16 secondaries, and achieved a range of 3874 yards.

Originally posted by Zalgiris 1410:

and on the other is that when they captured German 81mm mortar rounds they had to borrow from the Americans their 81mm mortars to fire them because the rounds did not fit in their own 3" medium mortars at all.

I should be interested to see your source for that. If it really says that an 81mm bomb won't fit in a 3-inch mortar barrel, it is simply wrong.

I suspect that it just says that the German and Italian rounds couldn't be fired, which would be correct up until the introduction of the Mark 5 barrel, which had a modified striker stud to allow it to fire German and Italian 81mm bombs [source: Jane's Infantry Weapions, 1975, Ed F W A Hobart].

People who still refuse to believe that the 3-inch mortar tube is 81mm in internal diameter are cordially invited to take calipers and ruler and measure a real one, as I have done.

All the best,

John.

[Zalgiris 1410]

Originally posted by John D Salt:

I should be interested to see your source for that. If it really says that an 81mm bomb won't fit in a 3-inch mortar barrel, it is simply wrong.

I suspect that it just says that the German and Italian rounds couldn't be fired, which would be correct up until the introduction of the Mark 5 barrel, which had a modified striker stud to allow it to fire German and Italian 81mm bombs [source: Jane's Infantry Weapions, 1975, Ed F W A Hobart].

That might account for some of the examples that I had in mind, captured 81mm mortar rounds not fitting in British 3 inch mortars because of firing mechanism not size. I can understand that. Any idea on when the Mark 5 barrels with the adroit striker stud were introduced and distributed?

I can think of two examples off the top of my head, firstly in "Tobruk", by Chester Wilmot and possibly also from in "Tobruk, the Birth of a Legend", by Frank Harrison somewhere where the Aussies complain about being out ranged by Axis mortars, although thinking about that one it might be more a case of a comparison between 51mm to 81mm mortars. Quite a few others might also have this confussion to them as well, may be I'm also thinking at least of a British Divisional histroy book or two by Patrick Delaforce?

A very good exampe I have comes from "Cassino, Portait of a Battle", by Fred Majdalany, which incidentally is a very good book on Cassino IMHO. During the 3rd Battle of Cassino the 4th Indian Division had to borrow some Texian US 81mm mortars to fire the large stockpile of captured German 81mm mortars rounds nearby specifically because they couldn't be fired from their 3 inch mortars. May be this happenned before the introduction of the Mark 5 barrel?

Thanx John for that detail.