[B.T.]

Oh no, it's that old optics thing again.

But I am really uncertain about several facts concerning the variants of the M16 in the T2k universe.

That's what I think is correct:
The standard rifle for all the US combat troops is the M16A2. I assume, a small contingent of M4 carbines has been issued. The majority is of the first production type, with the new handguard.
Another model would be the M4 or M4A1 MWS, the one with the rails.
The SOPMOD block 1 is in use, but only in very limited numbers.

I've read several things about the M16A3: Now, what is it? Is it a M16A2 variant with the full-auto option but with a fixed rear-sight/carrying handle, or is a flat top?

Next question: When have the M16 MWS variants been fielded? Or when were the rails for the long barrel rifle variants put into production or use?

I am well aware, that a lot more variants would be in use, especially the M16A2 carbine variants (M-733, M-723, maxbe even older XM-177En). Civilian versions will also be encountered.

How easy (or not) would it be, to equip the older versions with the A1-style handguards with the parts of the SOPMOD kits?
Would all the M4 variants be flat-tops?

I know, it is a very complicated issue, so I could really use a little help on this.

Any constructive income is welcome!