The Abyss

The weapons seen in this movie are

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MAC 10 with extended barrel

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The one in the movie didn't have a stock, though.

SIG-Sauer P226

Lt. Hiram Coffey (Michael Biehn) carries a SIG-Sauer P226 9mm, the standard issue pistol for the U.S. Navy SEALs.

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SIG-Sauer P226 9x19mm

I would need to see screencaps to be sure, but I remember the gun being a P220. The P226 may be used by the SEALs in real life, but the prop weapon used in the movie could have still been a P220.

The only good shots of the gun itself are pretty dark, so it is hard to tell the diameter of the barrel; but, the rounds they show during the scene when Ed Harris tries to subdue Michael Biehn are clearly 9mm FMJ's.

The diameter of the bore isn't necessary to tell if it's a P226 or not; in fact, it would be irrelevant because the earliest P220s were also 9mm. I would need to see if it has the double-stack creases on the sides of the frame. That's how you can tell a P226 apart from a P220. -MT2008

M16A2

Or maybe M16A1 with A2 handguard. It is seen weilded by the Bethnic Explorer's Armed QM at the scene when the underwater alien palace rises from the sea.

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M16A1 with A2 style handguards - 5.56x45mm. Not utilized this way in the real world, putting the distinctive A2 handguards on the older rifle was a method used by Movie/TV armorers to simulate A2 rifles when M16A1s were the only ones available.

I was watching it earlier n it was clearly a M16A2. The Winchester