Hey, I carried all the GAU variants in my 20 yrs in USAF CCT. When I retired in 95, we were just in the middle of upgrading our old Vietnam era receivers to the 14.5" NATO 1/7 M4 barrel. Some were restamped GUU-5P some were not, some got A2 uppers some didnt, I hear that they now have completely new guns, but if they need to be firedt, thier radios must not be working quite right!
IIRC, the GAU-5=10.5" bbl, GAU-5A= 11.5" bbl and the GAU5A/A = 14.5" bbl, all 1/12 twist pencil barrels
GAU meaning Gun, Aircraft Unit