Rob makes a couple of good points. I have also (infrequently) experienced buffer detent failures, but if the failure is complete rather than intermittent then you may not be able to break the rifle open by pushing the rear takedown pin open as the buffer may be too far forward and not allow the upper assembly to rotate/move.
In this event you'll have to slide both the rear AND front takedown pins open and remove the entire upper assembly. The buffer and spring will snake forward when you remove the upper and you will be able to push it back into the stock but it won't retain the buffer in place if the detent is trashed. Easy to diagnose that one.
I know at one time we used A1's that were upgraded to A2 standards, at least that is what the CATM instructors would tell us. As for the buttstocks I remember shiny black ones, and flat black ones, as for the cleaning kit compartment, I honestly can't recall if any of the rifles had them.
I don't think the Air Force was at all unhappy with their M16 & M16A1 rifles (remember, the M16 was Curtis LeMay's darling), but when the Army and Marines upgraded to the A2 rifles the Air Force had
to partially upgrade. The reason was that the military was standardizing on the M855/SS109 62 grain bullet and the 62 grainer would not reliably stabilize in the 1-in-12" rifle twists of the M16 and A1's that were designed for the 55 grain M193 cartridge. The A2 rifle uses a 1-in-7" twist that provides stability for both the M855 ball round as well as tracers, and 55-grain M193 ammo works fine with the 1-in-7" twist as well.
The Air Force chose a sensible and economic plan of upgrading at least some of their M16 and M16A1's by replacing only the upper receiver barrelled assembly and keeping their old lower receiver assemblies. This gave them new barrels as well as the ability to use the new ammo at about half the cost of new A2 rifles.
The old upper assemblies (minus bolt carrier group & charging handles that went into the new A2 uppers) were dumped on the surplus market much to the delight of many of us. I bought five of them myself, and the A1 retro rifle I built and posted about earlier is sporting one of the Air Force surplussed upper assemblies. I used another one to build a lightweight 20" rifle with a carbine buttstock. Win-win for the Air Force and me. Gratuitous pics below:
As to the buttstocks, the M16 rifle did not have a cleaning kit compartment but the M16A1 (and A2) rifles did have the recess. Easy to tell the difference between the M16 and A1 buttstocks because the former's rear sling swivel was moveable whereas the A1 had a fixed sling swivel. Also, the butt on the M16 was solid; the A1 had the trapdoor access to the cleaning kit recess. Shiny black was more common with M16's whereas A1's tended to be more flat black, but I think both flat and shiny versions of both exist depending on which contractor made them. I imagine that if you did not have a dedicated rifle assigned to you that you sometimes signed out a M16 and sometimes signed out a M16A1. Just speculating there.
Last observation and then I'll shut up... You note that your rifle has the HBAR barrel. I think maybe
the larger mass of the HBAR allows more vibration to be transferred to the rear of the rifle and may increase the perceived vibration. Not positive about that but the "noisiest" AR I have is an A2 with the heavy barrel. I just haven't futzed around with HBAR A2's enough to know whether increased vibration is endemic to HBAR's or merely coincidental. I suspect it is but am just not positive.