A Smith and Wesson model 39 modified with a shoulder stock, a locking slide and a suppresor. This weapons was designated Mark 22 Model O and used by the Navy SEaL's in Vietnam. Only a few undred were made and were mainly used for disabling sentryies, killing guard dogs and shattering lights. THe slide locked for maximum noise reduction and it's suppresor limited the effective range to around 100 yards.
That guy has marks on the slide of his hush puppy that represent the numder of sentries he dispatched back in 'nam.