In WW2, what was the barrel-change rate of the Bofors 40-mm?

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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.

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