Commercial Bullet Casting
For a couple years I cast muzzle loading round balls and conical bullets commercially. We did not do it full time but we did go through about a ton of lead per month, maybe a bit more. We used a Bullet Master casting machine that used 10 or 12 (I can’t remember the exact number of molds) double cavity molds. Due to the stresses imposed by the process the machine required steel molds. Mostly we used Lyman molds. I also hand cast things like Minnie balls that the machine could not handle.
In general I liked the Lee molds because they heated well and if cared for lasted a long time. When loading molds into the Bullet Master I inspected each mold looking for the same issues highlighted by Rigmarol. I did not smoke the molds but spent a lot of energy ensuring that the molds were clear, did not have rust, and the vent groves were clear. If bullets stuck in the machine it was likely due to the molds being cold.
Although tin may make the bullets cast a bit better I never used anything except virgin pure lead. A slight amount of tin can harden the lead enough to break the loading lever on a 51 colt style pistol and conical bullets load and shoot better when cast with pure lead.
In general for my use now I prefer the Lee molds because they are more reasonable, work pretty well and have a smaller sprue.