My barrel vise is made from two 1 1/2" square steel bars, each with a semicircular 1 1/2" cutout for split 1 1/2" diameter aluminum bushings. The bushings are made as needed, bored to size, then split. The blocks clamp together with 7/8" bolts. I sprinkle resin on the barrel where it is grabbed by the blocks. Pulled up hard, barrels do not slip. I would not be too optimistic about wooden blocks clamped by a machinist's vise.
Open end wrenches are widely used on M-14/M-1 receivers. Haven't heard of one being spoiled. I prefer a contoured wrench, fitted to the receiver.
Sometimes you cannot have too long a cheater on the wrench.