I have a set of barrel vise blocks that consist of two cast steel halves about 4" long 2 1/2" wide that when the 2 halves are mated together there is a 1 1/2" dia hole lengthwise through the blocks. To use, you put the barreled action vertically in your bench vise, poke a hole in a piece of cardboard and slide it over the barrel down to the top of the vise, now take the two halves of the barrel vise blocks and set hem on top of the cardboard on the vise, cut 2 strips of cardboard big enough to extend outside the blocks and place 1 strip of cardboard between each half of the block and slide it against the barrel, now secure this assembly with a C-clamp. Now you melt enough lead in a pot or ladle to fill the cavity and pour it in. After the lead cools, remove the C-clamp and the cardboard, and you now have a set of blocks for your barrel vise. Removing the cardboard strips allow for some squeeze in the vise. Some masking tape on the bottom cardboard and the barrel will reduce molten lead leakage. I have had these since the late 80s and used them many times, remelting the lead over and over. I don't know where you can find them to purchase, but they are pretty low tech to reproduce.
I could probably get some pics if that description is lacking