To be fair, the Small Arms Combat System (which I presume you are referring to) does describe itself as "a stand-alone game or as a combat system for any other game", rather than as a RPG.
For those not familiar with the Phoenix Command line, they did also produce some RPGs (e.g. Aliens, and Living Steel) based on simplified versions of their Small Arms Combat System. I did once bolt on the SACS with the Living Steel weapons and powered armour supplements to a game of Traveller, but only ever ran it once - it was just too slow in play.
Ironically, they designed and produced an Aliens boardgame, using an even more cut-down version of the SACS, and it worked really, really well. It was fast and exciting, with important decisions to make every turn, e.g. trying to judge whether to use all your actions to have a good chance of hitting an alien, but risk being left behind, or take an action to move and an action to shoot with a much lower chance of hitting.