Here's your answer - there were THREE variations on the 76mm gun as placed in the M4A1 76mm Wet Stowage, and other vehicles with the 76mm gun (M18 Gun Motor Carriage, and M4A3 76mm Wet Stowage)
76mm Gun M1 - plain muzzle
76mm Gun M1A1 - muzzle was threaded for a muzzle brake, but the threads are covered by a steel protective ring.
76mm Gun M101C and M1A2 - double-baffle muzzle brake fitted to the end of the muzzle. I think M1A2 had a chance to the rifling inside the tube, but that doesn't concern modelers.
BOTTOM LINE: K&C's M4A1 76mm Wet Stowage is modeled with an M1 gun, while the HB tank has an M1A1 gun. HB probably should have had an M1 gun as it is based on earlier tanks, but there were replacements of worn or damaged guns, so no big problem.
Gary B.
resident Shermanoholic