5.56 has been abandoned in favor of 6.5 caseless ammunition. 6.5 is the new standard in-universe, though not as universal as 5.56 is nowadays.

Also, the Idea of the MX is that you have multiple rifles that all use the same ammunition for greater logistical efficiency and ammo commonality inside and between Units.

NATO now have TRG-20 and TRG-21.Now they uses 5.56
Also DMR must have another kind of 6.5 ammo,heavier,powerful and much more accurate,like MK262 ammo for MK12 SPR.Or uses 7.62.

I cant see using a carbine with a 6.5mm round. The Ak-74 (where the ammo was changed from the AK-47)still uses smaller rounds than the carbine. I understand how the new nato rounds are supposed to be 6.5 but in reality you're going to want smaller ammo with less recoil, especially for a shorter barrel. The use of smaller ammo has been implemented in most standard rifles currently used now. That is even if previous models had larger caliber firing systems before.

edit- This cartridge was developed in around 2002 to provide Ar-15 type rifles with long-range capabilities and terminal effectiveness that are generally not possible with standard 5.56 mm / .223 ammunition. The 6.5 Grendel is based on .220 PPC cartridge, developed for shooting matches, which in turn is based on Soviet 7.62x39 catridge case. The 6.5 Grendel provides longer range and better terminal effectiveness over the 5.56x45, thanks to heavier bullets with better balistics. The price for this improvement is somewhat stronger recoil (although it's still noticeably less than that of 7.62x51 / .308), and decreased magazine capacity (due to a larger diameter case). The 6.5 Grendel is viewed by many as an 'ideal compromise' assault rifle round, although today its practical use is limited mostly to civilian applications (hunting, target shooting, self-defense). This cartridge is manufactured in USA and Serbia, and weapons for it produced mostly in USA.

^this would not be a realistic decision for short range weapon systems.

exist SCAR-H(7.62x51)in CQB version.Complitly not CQB veapon,but they're exist...
also OPFOR uses 6.5 in any rifle too(yet).
For purpose of usage 5.56 NATO have TRG-20,TRG-21.They also better in CQB,cause bullpup are more compact.

Also - exist logistic problems.Easier to carry one type of ammo,that one for rifle,another for DMR,third for LMGs,Forth - for |long continious list|.

About stopping power - well,in CQB this is more important thing,that at distance.Many Police SF uses more 9mm SMGs,because their round have better stopping power..Much more important thing at distance - ballistic characteristics,may be thats why future ammo is an compromise between 5.56x45/5.45x39 and 7.62x39.

Mass closing ancient tickets with no activity for > 12 months; assume fixed or too trivial.

If this issue is still relevant in current dev build, please re-post.