Saco Defense model 683 prototype SMG: a near-copy of the Brazilian Uru

Key points: Saco Defense model 683

• The 683 exists because of JSSAP, the joint service small arms program best known for the XM9 pistol trials, which also drifted toward a standard submachine gun on requirements Ian calls self-contradictory: a low 450–600 rpm cyclic rate plus a very fast burst option, and easy conversion between 9mm and .45.
• Saco Defense obtained a Brazilian Uru — a 1977 design built around only 17 components — and copied it closely enough that the two guns are hard to tell apart on the table; the parts are not actually interchangeable, since barrel thread pitch and feed ramp profiles differ, though the bolts will swap between receiver tubes.
• The carry handle is a sight, not an optic: a hollow tube with a single crosshair at the front and no rear element, aimed by centering the crosshair's circle inside the tube, and zeroed by loosening one screw and wiggling the whole assembly — a job that plainly wanted a factory jig.
• Saco fed the gun from Uzi magazines rather than the Uru's MP40-based mags, but gun 101's catch sits at the rear of the magwell and needs a cut added to the magazine; guns 102 and 103 move to a side catch that uses the standard Uzi slot, so unmodified Uzi mags drop straight in.
• The whole gun is two tubes, a drop-out lower and a barrel held by a front nut, so barrel assemblies swap in seconds — the route to a .45 version or an integrally suppressed barrel in the manner of the M3 grease gun, had it ever reached production.
• Gun 102 replaces the Uru's single component that serves as trigger, sear and semi-auto disconnector with a conventional multi-part group whose disconnector bar catches the bolt farther back rather than resetting the sear; Ian's read is that this followed a legal threat from the Uru's manufacturer after Saco showed the copy to its designer.
• Only three model 683s were ever made and all three now sit at the IMT — the program went nowhere, which Ian treats as unsurprising given how unachievable the JSSAP requirements were.

About this Gun Review video

Saco Defense model 683 prototype submachine gun: Ian McCollum field-strips serial numbers 101 and 102 at the Institute of Military Technology and shows how closely Saco's 1983 JSSAP submission copied the Brazilian Uru — same two-tube receiver, same open-bolt blowback, same drop-out lower. Saco's additions are a sliding stock, a carry handle doubling as a hollow-tube sight with a single crosshair, and Uzi magazines. Guns 102 and 103 drop the Uru's one-piece trigger-and-sear for a conventional fire control group, most likely after a legal threat. Three were built; all three are at the IMT.

Transcript: Saco Defense model