Hopefully this info is permitted here. You probably are all well aware of this but for the one or two who aren't:
If anyone has an original steel or bronze line-throwing cannon, most of those were made in 2.5-inch bore. A Coors beer can (not ordinary beer can, not soda can, only Coors beer can) fits perfectly and you can fill them with plaster of Paris or concrete. We've used good-quality anchoring cement because it has a pouring consistency like maple syrup. We use a can opener and remove the entire top to make filling easier.
If you can lay your hands on an old line gun, you really can't find anything stronger short of military artillery pieces. They were made to project a 17-pound projectile using several ounces of black powder, so when you use a one or two-pound Coors can of concrete, and a reasonable powder charge, you are well on the safe side.
These guns do need some adaptation for target shooting since they don't come with precise elevation adjustments.
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