I'm working on a one-shot SEAL's "adventure" for my next trip to Iowa.
An important characterization is weapon preference. Men would become attached to "their" weapon and retain it even if it had been officially replaced.
It's appropriate for the characters to have an opinion that one or another variation are superior to the others. The difficulty is expressing to the players why they should have such opinions.
Some would abandon the old gun and grab the latest and greatest (sometimes regretting it when it proved to be an unreliable experiment.)
A problem I have is that in late '67 a SEAL platoon would have their choice of five versions of the M16; The green-stocked AR-15 R601, black-stocked AR-15 R602, XM16E1, M16A1 or an M16A1 equipped with an HEL Mk 4 suppressor. Except for the silenced version, they're identical in game terms.
In real terms there are many minor differences that would matter to the user. The R601 and R602 do not have captive front take-down pins. That means it could be lost.
The XM16E1 and M16A1 introduced the forward assist. There are strong opinions to this day about the need and utility of this device. Some regarded it as, at best, a needless complication, some even considered it an accident waiting to happen.