Just a few off the top of my head...
(pronounced gar [like the fish] fifteen)
(Like the longer barreled MK16)
Gi "bigger stick"
G15 combat blowback "The Machine"
(To keep the military tradition of naming guns after their year)
I think that "GAR" would make a great series name because it makes it obvious it's in the g4 line, plays on the AR series name, and is associated with a predator that's both fast and deadly. Not to mention that Gar sounds cool.