This was an intentional change as reflected in the changelog:

http://forums.bistudio.com/showthread.php?149636-Alpha-Development-branch-changelog&p=2399989&viewfull=1#post2399989

"TRG now uses correct 5.56x45mm ammunition"

Still, there is something mixed up with TRG and magazines' configs, as observed in Dev build 0.59.105679.

TRG shoots 5.56x45mm ammunition and uses 5.56 STANAG mags, but:

• TRG uses 6.5 suppressor;
• TRG is described in inventory pop-up hints as chambered for 6.5x39mm;
• 5.56 mag is described as used in Mk20 and SDAR, but not a word about TRG;
• short rectangular 6.5 mag is described, as used in TRG;
• TRG doesn't accept any of the currently available 6.5 mags (short mag, Katiba mag, BLUFOR Stanag mag).

Yes. what he said. ^

Since I didn't see the reference in the change log and I saw all that armapirx listed above, I assumed the new change to 5.56 was a mistake.

da12thMonkey: Thanks for confirming that it indeed is a purposeful change.

Confirmed: The unit will rearm with 30Rnd_556x45_Stanag when rearming from a vehicle or ammocrate containing 30Rnd_556x45_Stanag.

However as armapirx states, the notifications are incorrect in the Inventory UI Dialog. Hovering over the items lists incorrect information.

It's not a mistake, all those changes were made to the MXM too, but from 7.62 mm to 6.5 mm. It started with the gun using the 30 rounds mags and the descriptions and markings were changed later. The patches will address these issues (sooner or later).

Are you guys dumb? It is NOT a mistake by making it 5.56, but it was a mistake that it uses 6.5 supressor etc. Also the developers had in changelog: "TRG now uses correct 5.56x45mm ammunition".
Search in changelog before saying things like this.

@pops - changes with MXM are bad IMO.
Of course IRL USA use same caliber DMRs such as SAM-R,but they as well have better optic and matched rounds.
SOS scope not calibrated for small arms,they calibrated for heavy calibers.
There is no sense between MX and MXM.Put HAMR to MX and it will be same as MXM.

Also Tavor now have 5.56 caliber,this is not a bug.The bug is - they uses 6.5mm supperssor

@samogon
Of course there are many inconsistencies with scopes atm. Similar with suppressed vs unsuppressed weapons (same ammo, same ballistics, different damage). Btw I see the MXM more in a role like the Mk12 SPR. Different accuracy and slight damage adjustments would be nice but this is going too much ot. Sorry for that.

@pops.Well Mk12 SPR in arma has beatuful optic.Lack of damage compense with high accuracy and great magnification(in ArmA1 it was best AFAIK).Here it's not work,this is just rifle like MX,but with diffirent 3d model unfortunately.

TRG family weapons are 5.56x45mm as mentioned in http://forums.bistudio.com/showthread.php?149636-Alpha-Development-branch-changelog&p=2399989&viewfull=1#post2399989
Arma 3 Beta contains correct magazines, ammo and weapon accessory for all of them. Could you, please, confirm that and close the issue?

Mass close.