The idea that there is so much UHMW out there that it should drive ammo selection is pretty questionable. There are far too many ceramic plates out there to think M855 will make a difference if you go up against body armor.

All the barrel twist arguments and longer range stuff is just not relevant for putting up a case of ball ammo.

Buy a case of M193 so you don’t have any training/range limitations. Then work on a couple hundred rounds of something barrier blind, and training. Some training is far more likely to help you survive a gunfight than a basement full of ammo.