What has worked for me is making sure I use the set screw in the action wrench (I also use Jim's from NSS) just threaded into the action to prevent slippage and then tighten the action wrench down with a slightly oversized box wrench... to prevent the barrel moving while tightening the nut I generally close the bolt on the headspace gauge and leave it in place once I have set the headspace... then torque down the nut. While I probably should use a barrel vice, I haven't. I've found that using the action wrench and slipping the handle down into a hold drilled into the top of my work bench (specifically drilled to hold the action wrench handle) I haven't had any movement/slippage problems.