I am building a barrel vise. If you have one how far is it between the bolts. I will be using 1" by 3" flat for the bars. I am using oak for the jaws and will also make alumin. ones if needed. might have to buy the alumin. blocks but you get the pic.
Space the bolts just far enough apart to get the barrel in while leaving room for some meat on the jaws. There is no set spacing, especially on a home built vise.
It is easier to use slightly larger jaws for smaller barrels than to stretch the bars.