@ Gus.... this is indeed in .45ACP and not in .303
Just looking at the bor in pic 3 gives you an impression
of the caliber. ( surpressing .303 would be eh... hard)
I though of the DeLisle as well after posting here,
but what we have here i dunno.
@sunray of course the scope isn´t G.I,
but the gun could be.......