On a point of pedantry the M16A1 was not originally adopted by the USAF it being a model drawn up specifically to US Army requirements.
The AR-15 having been trialled by the USAF and US Army advisors in the very early 1960s was first adopted by the USAF as the M16. That variant had an early 3 prong style flash eliminator, "fenceless" lower receiver and no forward assist as some of its main distinguishing features.
The first model adopted by the US Army was the XM16E1 which saw initial service in 1965. This had a number of changes to it as requested by the Army. The most noticable was the addition to the right hand side of the upper receiver of a forward assist plunger and a partial "fence" on the lower receiver in front of the mag release catch. The rifle retained the three prong flash eliminator and the short butt stock with articulated sling loop from the AR-15 & M16. The most important yet invisible change demanded by the Army was a change to the propellent used in the ammunition in order to increase the muzzle velocity. This had a negative result on the rifle's performance. The change in propellent type caused increased carbon fouling in the rifle which was compounded by the fact that initially no cleaning kits were issued to the troops as there was an unfounded claim that the rifle was "self cleaning". The increased muzzle also arguably reduced the projectile's leathality, the higher speed in conjunction with the twist of the rifling causing the projectile to become too stable to the point that on impact they tended to pass cleanly through the target without tumbling and giving up energy into the target.
In 1967 a product improved version of the XM16E1 was standardised as the M16A1, it having many modifications notablely an enclosed "bird cage" flash eliminator,a "full fence" lower receiver, a change to the twist rate of the rifling, an improved lengthened buttstock with fixed sling loop and a butt trap to accommodate a cleaning kit. The M16A1 had become the more common rifle found in US service by 1969.
Throughout their usage of the early M16 series the USAF never formally adopted the features requested by the US Army, many USAF rifles being easily identifiable by their lack of a forward assist plunger which was always of questionable utility.
Take a look at Retro Black Rifle for a breakdown of the features of the various models of AR15/M16 rifles.
You may just pick up that I'm a gun nerd! 😁
