Tough one. Since it is on the USAF feed, I assume it is select-fire mil spec and probably full length barrel, but can't rule out it being an M4. It has a picatinny rail instead of the integral carry handle so it's an A4 model. The markings are too blurry to see, but probably a colt and probably 5.56mm. I'm not sure the airforce has any piston driven types vs the standard direct impingement and I'm not sure if they have anything chambered in the 6 mm or 7.62 that some spec ops use. Be helpful to see the bolt group and feed ramp to tell more specifics.