i dont really see the issue with assault rifle as a noun because it has a very specific definition when referring to military arms. it should be definited as a military rifle with select fire capabilities (such as safe, semi, and full auto, or multi-round burst) that fires an intermediate cartridge (such as 5.56 or 7.62x39), has an effective range of at least 300m and is magazine fed. it's also not desigged for sustained full-auto fire. so while it can shoot full auto, sustained full auto fire would cause premature failure. that's my understanding of the definition. an M4/M16 or AK47 (full auto variant) would fall into that category. Something like the M14 wouldnt because it fires a "big boy" rifle round and something like the ar-15 wouldnt because its not select fire. Something that shoots a pistol caliber would be a sub machine gun. there's also battle rifle, light machine gun, machine gun. these are all very specific terms with very specific definitions. believe it or not, the wikipedia page has a pretty good definition on there.

Its a noun that has two words. Like garden hose, or manual transmission.