Couple of other things about molten lead. It melts a scootch over 620F to keep it molten enough to pour it needs to be around 750-800F. If you pour molten lead into a clean metal container it will stick like glue. For lead to come free from steel or aluminum it needs a coating of carbon like you get from burning a match under steel or aluminum leaving soot behind.
Next problem is your spot on about shrinkage. It will shrink as it cools giving incorrect readings.
Next problem is whatever you plug the bore with will melt or burn under the 700+degree heat. Instantly.
Next problem is that it only takes about 400-600F of heat to start damaging hardened steel through annealing. Pouring 700F+ degree molten lead IF you can pour it without it silver dragoning yourself, IF you can keep it n the barrel, IF you can get it out again, IF you can pour it without damaging the barrel steel, IF you can get it to mold without shrinking you have worked a miracle.
It only takes a tiny drop of water or oil or any liquid for the dragon to appear. In the video below he is tossing water from a long way off into only a tiny bit of lead. If you get that stuff on you it will eat you to the bone. Not something to mess with lightly.