BTW “XM” is simply indicates something experimental. Example, the designation for the blank round for the M16 was originally the “XM200”, then changed to “M200” when approved for production.
The US changed to this in the late '50’s, I think - up until then, the designator was “T”.
The “T/XM” and “M” designators are used on everything, not just ammo.
Experimental variations of previously standardized items are usually labeled with an “E” suffix, i.e. M16E1. This would be changed to “A” for improved items that go into production.
This is not a hard-and-fast rule. The .50 “M2HB” stands for M2, Heavy Barrel. Not that I’ve ever seen the light barrel!