The M68E1 in the M60A1 and M60A3 is has a 50.92 calibre (210 in) bore. It is 52.84 calibres (218.5 inches) from the muzzle to the rear face of the breech.
The M68A1E4 is a low recoil modified version of the M68A1.
The M68A1 was developed for the M1, and has a total length of 236.5 inches (1.5 feet longer).
So, 236.5 inches is 6007.1mm, divided by 105 equals 57.2 calibre from the muzzle to the rear face of the breech.