The S&W Model 76 was a close copy of the Swedish Model 45 Carl Gustav SMG.
During the Vietnam war, the "Swedish K" as it was known, was a favorite of various special operation units, especially when fitted with a silencer.
During the war, the only SMG's readily available for use in those conditions, were the US M3 "Grease gun", the British Mark 2 STEN gun, the MAC series of "machine pistols", and the Swedish Model 45, all of which had silencers available for them.
The M3 and STEN were old designs, and the MAC fired at such a high cyclic rate it just wasn't suitable, so the Model 45 was the preferred gun for special forces use.
At some point, the Swedes got all huffy about the Vietnam war and refused to sell the Model 45 to "belligerents", meaning us, since the communists didn't buy any.
To provide a replacement, S&W produced the Model 76.
It was used most heavily by SEAL teams, and some were sold to civilian police departments.
When the Navy failed to buy more than a few for SEAL use, and no other market appeared, the Model 76 was discontinued.
Today, the S&W Model 76 is a collector's item.