The FAS-173 was developed in 1973. It was based on Foote's earlier FAC 70 carbine, scaled-up to take 12 gauge shotgun shells. It was designed to be conceptually similar to the AA-12 shotgun developed by Foote's friend Maxwell Atchisson. Only hand-made prototypes of the FAS-173 were ever developed.
The FAS-173 was a gas-operated automatic shotgun utilizing a basic long-stroke gas piston. Like most of Foote's designs, the FAS-173 was designed to be as cost-efficient as possible, utilizing cheap steel stampings to comprise most of the external components. Prototypes were built in 12 gauge and 20 gauge, and were fed from 10-round box magazines.