The Advanced Combat Shotgun, or ACS was made to a Paternian Army requirement for a more potent combat shotgun than the existing pump-action Rocker Arsneal M100. Three companies entered; KRIEG, Rocker Arsenal, and Carlton Arms.

KRIEG was the most radical design. The Jackhammer an automatic shotgun with a blow-forward gas-operated bullpup action. Feeding from an 8-round drum, it was compact and powerful.

Rocker Arsenal was rather conservative, and submitted two designs; a variant of the semiautomatic M110 shotgun with detachable box magazines and full auto capability know as the M110T, and a straight-pull bolt-action shotgun designed to fit under the barrel of the M16 Rifle or M4 Carbine known as Commando Key.

The Carlton Arms approach, known as Advanced Combat Shotgun, was a more intermediate design, a gas-operated automatic shotgun whose basic receiver could be configured into either a bullpup or conventional shotgun, fed from box magazines.

In evaluation trials, the conventional ACS proved most appealing to testers, with the Jackhammer coming second, followed by the bullpup ACS, the M110T, and the Commando Key coming dead last. The high controllability of the Carlton designs meant that they were most capable, where the Jackhammer and ACS Bullpup's rear-heavy nature made it less so.

However, the Jackhammer, ACS, and Commando Key were most compact. The M110T was rugged and simple, partially due to the fact the M110T is a modification of a production firearm, where the others are prototypes. The Commando Key was the least liked due to its low weight and manual action working to increase recoil dramatically.

However, field trials were a different story. While all other entrants had an ambivalent reception, the Commando Key was received enthusiastically. This was solely down to a feature that was never accounted for in the trials; the Commando Key could be mounted under the barrel of an assault rifle.

As the main role of a combat shotgun was for door breaching, and CQC, the light, compact Commando Key was well-received. This means that while the other options require the soldier to carry either them or a rifle, the Commando Key allows them to carry both a shotgun and an assault rifle, making the individual far more versatile.

As a result, the Commando Key was adopted. Despite this, Paternian Scout Rangers found the weapon to be the most effective shotgun with grenade rounds. With grenade slugs, it became a miniature automatic cannon.