This is from gunthings.com. The description below it says:
"We sell a similar vise on our FNFAL / L1A1 Tools webpage. Forming the lead vise blocks: Block off the ends of the channel iron with wood. The lead (use soft, pure lead) is melted and poured into each channel iron separately. Place a cylindrical object which is slightly smaller than the barrel to be installed into the melted lead. It should be recessed to half it's circumference. This procedure will ease the shaping to fit the barrel. After the lead has cooled, shape each block half to fit the barrel. Do not remove the lead from the channel iron. To use the vise, apply a dusting of Brownells barrel rosin to block halves and clamp the barrel between the block halves with a 6 holed steel plate on each side. Tighten the bolts in a pattern similar to installing an intake manifold on a car engine. The first time the vise is used, the lead will conform to the shape of the barrel as it is squeezed together by the steel plates. After initial use, the blocks will not change unless a different barrel contour is clamped. The tightened vise is either bolted to a very strong workbench, or held tightly in a large benchvise. Any barrels installed must be properly headspaced. Consult a gunsmith for any questions regarding installs. "