guns

Kinda of, but if you could SBR a FCU you could swap it between an Ar-15, Ar-10, or put a stock on a handgun. That's absolutely a perk of lower receivers but if it was the FCU that was the firearm instead it could be even more flexible. Currently you can SBR a Ar-15 lower but then you're stuck with just AR compatible parts, with a FCU nothing would stop companies from selling pretty much most types of firearms that accept that FCU.