I've rebarrelled several AR15/AR10 platforms and rather than spend the $$ on a vice block, I've always just used wood. A set of plywood ends on either jaw vice works perfectly when the receiver is placed with the top rail against the jaw. The receiver is structurally stronger in this postiion so it's uptake of the ensued load is much better distributed anyway. I've never understood why those vice blocks all place the receiver upright, looks like a great way to crack it if you accidentally over tighten your vice!
When using wood/plywood, tighten to the point you just hear the wood begin to ping and you'll be set to put your barrel on. PM me if you want some pics sent of what I'm referring to.
My .02, hope that helps...getting those receiver blocks can be a b***h since not many companies stock them.



























