[MarkSheppard]

According to R.P. Hunnicutt's SHERMAN, the two 75mm guns installed on the M4 Medium Tanks were:

75mm M2 gun, which had the following measurements:

Length of Bore: 28.5 caliber
Length, Muzzle to rear face of breech: 31.1 caliber

and was installed in the T6 Prototype, and the first couple of Shermans off the production line. These are identifiable via the concentric counterweights installed on the end of the barrel; this is due to the gun mount being designed for the longer M3 gun, so the counterweights were a 'hackjob' to make the shorter M2 gun work in a mount designed for the longer M3.

75mm M3 gun, which had the following measurements:

Length of Bore: 37.5 caliber
Length, Muzzle to rear face of breech: 40.1 caliber

and was installed on the massively overwhelming majority of 75mm Shermans (99.99999%) built.

NOTE: The U.S. measured “calibre” as the length from the muzzle to the rear of the barrel (bore), while the Germans measured it from the muzzle to the rear of the breech. Thus, the US 75mm M3 gun would have a calibre of L37.5 under the US system and L40.1 under the German system.

This may be the cause of confusion; someone saw two differing sources, one measuring calibre via the German system (L40.1) and another via the US system (L37.5) and thought there were two 75mm guns with different calibre lengths in Shermans.