On another note, it makes sense for a person (fictional character), if they had different duties, to have two different uppers for their AR, right? They'd have the same lower, would save the hassle of carrying more than one type of rifle ammo, and would have a pretty versatile weapon, right?

I think it is vary variable. Originally the Mk 12 SPR was just an upper kit and it was used with M16A1 or M4A1 lowers (M16A2 and M4 burst lowers have a worse trigger), and later it was made with a dedicated lower. I don't know but I would guess that these later ones were semi only. If you look at our stock images, the SPR prototype and the first Mod 1 image both use M16A1 receivers which would have full auto.

Do you mean on a mission, or in a country? If you mean in a country, then yes. For example, I know that the UK special forces have long barrel and short barrel uppers for their L119A1 carbines which they switch depending on the mission.

If you mean would it make sense to carry an extra upper on your back if you wanted to make a long distance shot or something, then no. Manufacturers over the years have tried to tout the idea of switching uppers out for different needs on the fly, but as far as I know it has never been done like this. You would be much better off just carrying a second whole weapon. You don't want to have to be stripping down your rifle in the field, and fumbling to get the bolt out of one weapon and in the other being defenceless the whole time.

By that, I meant, say a person normally carried a PDW 5.56, but then had to go on a mission.

Could that person feasibly have a second upper that they could swap out on at HQ, go into battle with the upper (It would go from a 10.5 inch barrel to a 14.5 inch barrel), then later on come back and put the PDW upper back on?

Or does it make more sense to just have two separate weapons?

In that scenario, yes it makes sense. British special forces can do exactly that, swapping between a 16" upper or a 10.5" one. It would only ever be something done by special forces though, every one else has 1 gun and makes do.

Gotcha.

Thanks.

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.

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.