Someone to Talk

[Excalibur]

Well, they wouldn't carry an extra upper while they are in the field. That's just not practical. Special Forces pick their loadouts very carefully before they are deployed. If it's a mission behind enemy lines, or direct actions, they'd need to carry all the essentials and still be effective without being weighed down.

You have to take into account that a guy isn't just carrying his primary weapon and ammo. There's his rig that holds his extra mags, plate carrier, a sidearm, pack with other shit inside like food, first aid, and other mission specific items, radio.

For weapons, a team would need to pick the right weapons for who gets to do what. If they need a guy with a DMR, a guy gets a long ranged weapon. If they are all going in doors, they all get shorter guns. It really depends. Special Forces guys are a unit. There are no solo commandos in reality.

[Yournamehere]

Excalibur's reasoning further supports why a vanilla M4 makes the most sense; it does everything pretty okay rather than some things better than others.

Swappable uppers would be cool, but it probably wouldn't be something he would carry with him and swap at will, especially with your application parameters. He would be just as well suited with one gun than with one and a half. If your range increased, then I would recommend something else, but with your parameters, one upper is fine.

As for Commando's comments on 6.5 and 6.8, in order to run those cartridges, you'd need a custom gun or an obscure rifle chambered for them, and a supply of magazines and ammunition which, again, oversteps the parameters of the character to no real benefit other than having a semi-snowflake gun (which isn't ideal, especially in a lone wolf scenario). To that same point, even with 6.5's existence during the timeframe and 6.8's fielding, I highly doubt that they were ubiquitous enough to be effectively obtained by the character in large quantities, whereas 5.56 was, is and always will be.