In his book, 'Modelling the US Army M4 (76mm) Sherman', Steven Zaloga refers to the paint on the rear portion of the gun barrel being worn away, caused by the repeated recoil action. Would this have been a common feature?

Either worn away or never painted at all. The clearance was very tight, and this area is usually bare metal.

The rear part of the barrel is a cylinder known as the "slide" because it slides backwards in the cradle (which is a sort of tube on the M1 76mm gun) into the turret during recoil. The bare section is the front end of that slide area, which is normally left unpainted but would scrape itself clean anyway when the gun is fired. Paint it a silver colour.

Thanks for the replies ... as I thought.