Stay away from the PD stuff (of course, you don't have to listen to me).
I bought a bunch of the XM193PD (not XM855PD) from Sportman's Guide. When it showed up, I opened one box, dug a big handful out, eyed them, them starting sitting them down in a line on the table. I never finished standing them up in a row. They were so bad, it was hard to find 2 that were the same height (set back into the necks, bent, etc).
I knew there was no way I was going to run it through one of my rifles.
I dumped them back in the box, called Customer Service the next day, and asked for a refund.