Federal Gold Medal Match seems to have been the standard for a long time. Mostly as a method to determine if there was a problem with handloads versus a rifle when looking for problems or as a god starting point for determining accuracy of a rifle.
A round hitting almost 3 feet low seems really strange. Not impossible, but strange without another factor involved. You say another round wouldn’t chamber in an AI rifle. Was this the same rifle used for this entire box of cartridges? Did it chamber in another rifle or was there another rifle available to even try?
What’s the range you were shooting at like? Clear or are there any twigs, brush, etc... Unlilely events happen. Round hits a stalk of something growing and slows it.
How many rounds fired that day? Do you track ammo lot numbers? I know I dont.