This was a Winchester produced cartridge for the Ingram MAC-11 Submachine Gun.
Below is a description from the August 1975 AFTE Journal:
SPECIAL .380 SUBMACHINE GUN CARTRIDGE FOR MAC (INGRAM) SMG
By Monty C. Lutz, Chief Firearms Examiner
US Army Crime Laboratory
Fort. Gordon, Georgia 30905
The Winchester-Western Cartridge Company has recently started manufacture of limited amounts of special purpose .380 automatic cartridges. This ammunition is being made on special order for the requirements of the Military Armament Corporation, Marietta, Georgia and is being loaded into cartridge cases having normal .380 ACP dimensions. Even though the external measurements are the same, there are several great differences between these new cartridges and the normal .380 automatic round. The headstamp is marked “MAC .380 SMG” and this identifying feature makes them easy to distinguish from any other round.
The 130 grain bullet loaded into this round has s full metal case and is .355 inches in diameter. In those instances where only a bullet is recovered, the firearms examiner might be misled if he were to use only the “Table For Determination of the Caliber From the Weight of a Bullet.” Most of these tables list. .380 automatic cartridges as having full metal case bullets weighing 92-97 grains. In most instances, full metal case bullets, having a weight of 128-130 grains are assumed to be bullets designed for firing through a .38 caliber automatic weapons. The Military Armament Corporation .380 SMG cartridges are packed in boxes designed to hold 50 rounds. These white cardboard containers have very specific instructions and warnings intended to inform the buyer about the cartridges in each box.
These rounds of ammunition are loaded with 3 grains of flake powder, and because of the added length of we heavy bullet, the powder charge is compressed. Due to this feature, increased chamber pressures are developed, well within the limits of the SMG they are designed for, but with enough added pressures that they could become dangerous in an ordinary .380 pistol. Test firing of the Model M-11 .380 SMG revealed that the round attains a muzzle velocity of 950 feet per second and develops approximately 254 foot pounds of muzzle energy. With the increased muzzle energy, the MAG .380 cartridge surpasses any other commercial .380 automatic cartridge and makes a sizeable showing when compared to commercial .380 Auto and 9mm Parabellum ammunition.
DESIGNATOR CALIBER BULLET WEIGHT MUZZLE VELOCITY MUZZLE ENERGY
MAG .380 auto 130 grains 950 FPS 254 foot pounds
W-W .380 auto 95 grains 955 FPS 192 foot pounds
Super Vel .380 auto 80 grains 1,026 FPS 188 foot pounds
W-W 9mm Para 116 grains 1,165 FPS 349 foot pounds
Several hundred MAC Model 11 Submachine guns have been produced, with some going to various police departments in United States and others being sold to authorised agencies throughout the world. The Firearms Examiner may encounter evidence involving one of these weapons having fired standard .380 automatic or the special MAG .380 submachine gun ammunition. The following class characteristics for the Model 11 may assist identification.
Groove dia. Direction of twist Nr lands and grooves Land width Groove width
.3534 in. Right 6 .1145 in. .0651 in.