The AR-9 was developed in late 1955. It was due to go into production the subsequent year but was held back and developed as the AR-17. Only limited numbers of AR-9 prototypes were ever made.
The AR-9 was a recoil-operated, self-loading shotgun chambered in 12 gauge. It incorporated an early version of the AR-10's rotating bolt and was built, like most ArmaLite products, from aluminium and fiberglass components, rendering it particularly lightweight when compared to other shotguns of a similar type.