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The Swiss Arms SG 553 RP 4K Rifle features the ability to take AK-47 magazines and 7.62x39mm cartridges currently deployed with certain tac units overseas. The adjustable gas system allows the user to run at a level that works best for them.
The resulting Colt 9mm Submachine Gun retained the characteristic M16 lower and upper receivers and operating controls. The elimination of the gas system in favor of a simpler direct blowback design & the caliber switch to 9x19mm. Firing from a closed bolt greatly helped the SMG accuracy.