great idea. you printing with standard filament or the flexible (TPU?) stuff?
I had the same idea of making my own slugs and buckshot a while ago. bought the Lee mold and have a bunch of pure lead, some spare hulls and wads and maybe an MEC non progressive to play with but haven't had the time to do much past that. Maybe this fall I'll get some projects done and set up a casting area to smelt some lead and make some bars and then into some molds..
Using cheap PLA. The last print, I used a thinner base and top, which gives it a little flex. I may do the walls a bit thinner on my next attempt; it's still pretty ridged. Have no idea what a ridged PLA buffer will do to velocity and accuracy, or how the PLA buffer will hold up to the momentary stress of thousands of PSI before it gets going down the barrel. Hoping it will remain intact and I can examine it after firing. And, of course, I'll be patterning the buckshot.
For the lead stuff: Years ago, I put all of my casting gear on a cheap, Harbor Freight service cart. I wheel it out of the basement onto the back porch when I want to cast. Minimizes setup and cleanup.