That is only true for automatic firearms, and only partially true for them. Usually the modifications are reducing the strength of the springs, or enlarging th

That is only true for automatic firearms, and only partially true for them. Usually the modifications are reducing the strength of the springs, or enlarging the gas port. There's really no reason to ad "valves" or anything in the barrel its self. Given the time period of the movie, an automatic firearm is unlikely anyway, so I'd be inclined to think that this is not the case.