Corrected entry: When Coffey steals the sub to take the nuclear warhead down the trench the big bearded worker fires the Seal's submachine gun at the sub. When he does he holds the gun by the extended barrel that Coffey screwed onto it earlier. This extension a suppressor and has holes drilled all along the length of it. The holes allow gas to escape the barrel. That gas is extremely hot and would have burned the worker, but he doesn't even flinch.
Correction: Silencers only have 2 openings (one on each end of the tube - an "in" and "out"). You have probably seen silencers in movies with a guard around the outside that has ping-pong-ball sized holes in it. This is just a heat shield. You are correct that silencers get very hot - especially when used on a full auto machine gun such as in the movie. Hence the need for this heat shield. But it takes a few secs for the heat to build up enough to burn your hand.
Correction: The barrel extension is simply that - an extended barrel with a ventilated shield to allow the firer to use a two-handed grip on the weapon as an automatic weapon that small has vicious recoil, the first one or two rounds would be on target but all the others would decorate the ceiling. It is true that a suppressed weapon has holes drilled into the barrel but they are very small and are used to allow the combustion gases to expand into a CLOSED cylinder thus reducing the noise of the shot.