Have you tried any of the following brands/loads:

GGG M855 is real NATO certified ammo and very good ammo.

IMI M855 is the real NATO spec load and cartridge as well and it is very good

PPV M855 is the real NATO spec load/cartridge and it is not only high quality but reasonably priced

MEN 5.56 that you see in the US is NOT NATO spec ammo but it is very good quality and very consistent.

Having said all this if it does not matter to you that XM855 is not NATO spec ammo, then of course buy what ever ammo is priced right and good quality. Because I plan on my 5.56 ammo being in storage, I like to have ammo that is primer pocket and case neck sealed per NATO spec. The crimped primer is not a big deal to me but it must be important for battle conditions and keeping ammo stable and functional under adverse conditions.