So while the slugs I'm intending to use for base testing aren't quite the right size, they're standard diameter, but function well from my normal hand loads and get me decent accuracy on the cheap for practice. They're even just a touch undersized for plastic hulls because they use an attached plastic base to achieve the rifling effect of a more traditional foster slug. I'm hoping, I can use some kind of paper patching, thin enough that it kinda rips away once it starts rotating and the rifled base can engage better, or find some of the bases that are slightly oversized to center them in the casing better. I use them mostly because then I can use the felt wads I cut anyways for my normal slugs and as cushion in my shot loads, without the wads being pushed up into the base of the slug like you mentioned. Saves me money in the reloading process, I buy gas seals just because it gives me more confidence, of I could make them I would lol. And in testing, with all my loads before I nail down a good load, I always lean on the edge of caution, starting lighter and lower pressure and testing the concept before pushing for effectiveness. But, if I can prove to myself that I can make the concept work and can get the tools on hand to make something decent, I will definitely be hitting up machine shops for more purpose built equipment for the process, from shells to dies, I just don't want to sink too much cash into it before finding out it's just too far outside my knowledge base