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I was just reading about the 37-mm 70K Naval gun that the Soviet Navy used for AA. I wanted to see how it compared to the Bofors 40-mm. These were both guns used on naval ships for anti-aircraft defense.

They seem pretty comparable, except possibly for one thing. Wikipedia says this:

One drawback was that the 70K required a barrel change after every 100 rounds fired. To improve on this, a twin-barrel water cooled mount, the V-11 (called "W-11" in East Germany and Poland because of different Cyrillic transliteration), entered service in 1946, and was in production until 1957. A total of 1,872 V-11 mounts were built.

There was no citation for it. But...ouch, a barrel change every 100 rounds?

Then I went to the Russian version of the 70K wikipedia article. It clarifies:

Охлаждение АУ было воздушным и являлось неудачным: так у 37-мм ствола с воздушным охлаждением длина непрерывной очереди составляла лишь 100 выстрелов по сравнению с не менее чем 158 у ствола с водяным охлаждением. После 100 выстрелов ствол с воздушным охлаждением либо следовало заменять (на что требовалось не менее 15 минут), либо ждать его охлаждения около 1,5 часов.

Google translation:

The cooling of the AU was air and was unsuccessful: for a 37-mm barrel with air cooling, the length of the continuous queue was only 100 shots compared to no less than 158 for a barrel with water cooling. After 100 shots, the air-cooled barrel should either be replaced (which took at least 15 minutes), or it should wait about 1.5 hours for it to cool.

Again, it is uncited. But it does offer clarification on a few things.

So now my obvious question is how does this part compare to the Bofors? What is the barrel change rate of the Bofors 40-mm gun? I did look for this on google with various keywords like "Bofors 40 mm barrel change" and "swap" and "overheat". It all came up nil. I hope someone here has a good idea on how to find this info.

  • TheMalta Command page on the Bofors 40mm AA Gun doesn't give a set number of rounds, but describes the 'spit test': "Due to the high rate of fire, the barrel would tend to overheat. So the ‘spittle test’ would be performed and if the drops of spittle were seen to jump about, “change barrels” was ordered, Each gun had a spare barrel and this would be exchanged with the hot one to enable the gun to continue firing. Due to the intensity of the enemy’s air attacks, the static guns around Grand Harbour were issued with an extra spare barrel!" – sempaiscuba Dec 10 '18 at 0:49
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    "Ouch, a barrel change every 100 rounds?" -- Not that surprising. Take the "legendary" MG-42, where the recommended barrel change rate was after every 250 shots (one belt segment). And that's for 7.92mm, not 37mm. Of course you could fire more than that in a pinch, and probably would if under attack, it's just that the chances for failure and especially wear on the barrel skyrocket. (Which you don't much care about if the alternative is getting killed.) – DevSolar Dec 10 '18 at 9:15
  • A differential factor in barrel life would be a Stellite ( high temperature cobalt alloy) liner . I know some barrels had it but I doubt all barrels had it. – blacksmith37 Dec 10 '18 at 15:57
  • 37mm, 40mm and similar calibers usually only had about few dozen rounds in a magazine , so changing barrel after 100 would not be so unpractical. – rs.29 Dec 10 '18 at 19:12
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Terry Gander: "The Bofors Gun" reports that the 1930s land-based Bofors were air cooled, single barrel and supplied with two spare barrels to be exchanged by four men within a few minutes if overheating should occur. To be determined as needed. The naval version had water cooling from the start.

…Barrel life (rather than changing for cooling between periods of firing) was between 9,500 and 10,000 rounds. (p17)

Prolonged non-stop firing was possible if the loader could keep up eith the gun but too much fully automatic fire without intervals could shorten the life of an air-cooled barrel. (p21)

The WAR DEPARTMENT TECHNICAL MANUAL TM 9-252 40-mm Automatic Gun M1 (AA) and 40-mm Antiaircraft Gun Carriages M2 and M2A1 also remains vague – or practical if you like:

  1. GUN.

a. Barrel Assembly.

(1) Guns become copper-fouled to less extent when cared for in the proper manner. Wear in the bore does not depend entirely upon the number of rounds fired but also on the care given the bore in cleaning and cooling between periods of firing.

(2) Since the accuracy life of a gun is decreased by a fast rate of firing and the attendant heat, the gun should be allowed to cool and should be washed as often as practical. Barrel assemblies should be changed during firing whenever necessary because of overheating. Each projectile must be cleaned before it is placed in the rack preparatory to firing. The bore should be cleaned before firing.

Similarly, the NAVY DEPARTMENT BUREAU OF ORDNANCE – ORDNANCE PAMPHLET No. 820 – 40MM ANTIAIRCRAFT GUN October 1943 remains silent on average intermittent barrel change rate d caused by overheating due to sustained firing. Even though the naval version was typically water cooled, mitigating the effects of overheating, the instructions only warn that it could overheat, eventually.

Some points remain to be observed: the theoretically achievable rate of fire is somewhat reduced, as the reloading takes away some of that cadence and therefore automatically cools the barrel a bit. The water cooled version overheats much less overall compared to the air cooled version. The air cooled version is typically fluted:

enter image description here (Src: The Real Benefits Of Barrel Fluting)

And finally, compared to the Soviet gun: the barrels could be exchanged in 3 minutes instead of 15.

  • The quote from page 17 actually says Barrel life before the barrel had to be replaced due to excessive wear (rather than changing for cooling between periods of firing) was between 9,500 and 10,000 rounds. I think you should have quoted it directly because that parenthetical info is very, very important. The barrel change rate due to overheating is exactly what I'm looking for. – DrZ214 Dec 10 '18 at 0:34
  • I thought that it was mostly used by the Navy. – John Dee Dec 10 '18 at 3:05
  • @JohnDee the Bofors was extensively used by many nations on towed mounts and sometimes self propelled vehicles, just as the very similar Oerlikon. – jwenting Dec 10 '18 at 4:58
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    Check out archive.hnsa.org/doc/bofors40mm/index.htm for the US Navy's owners' manual for the gun. It describes a "water jacket" surrounding the barrel. – kimchi lover Dec 10 '18 at 13:26
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    You are welcome. On rereading your answer & my comments, I can only dredge up careless reading due to lack of sleep as an excuse. – kimchi lover Dec 10 '18 at 13:49

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