Guess it could be any 5.56 piece, not just AR's, but in my case it is AR's. I zero them all with Mk318 SOST. I'll use M855 to get it as close as I can then finish off the fine tuning with Mk318. That is not the only round I have in bulk but I figure that is the one I would load mags with first if it came to that. This is regardless of 14.5", 16", or 20" barrel lengths. I figure if I ran out of Mk318 I could re-zero what I was shooting with the next item in line.

How about you guys? Do you have "designated purpose" 5.56 weapons you zero and have a specific round allotted for, i.e. maybe a DMR piece that you use with 77gr (or similar) and something else for other guns? Do you choose the round by barrel length?