With "hardcast" alloys, bullet size/fit becomes a lot more critical. The reason I got into casting was because I bought various commercial cast products, and realized it was a crapshoot when it came to accuracy and leading.
Matching the alloy to the task helps a lot, as softer alloys obturate more easily, and thus will not allow as much gas blow-by, which is a usual cause of leading. Too-soft alloys also cause leading for the same reason, when pushed harder than the alloy can handle.
If buying commercial cast, you need to think about how fast you want to shoot them. Most handgun loads do better <15 Brinell, which is not classified as "hardcast." It helps to know what size works best in your gun, and hopefully buy that size. In your GP 100, that would be the size that snugly fits the chamber throats. And the "bevel-based" bullets typical of commercial cast, while making for easier loading, CAN allow gas blow-by, causing leading and fliers.
You may luck out, and the random bullets you buy work well. No way to know until you buy, load, shoot.
Last, I should say that powder-coated bullets cover a lot of sins, and seem to shoot better when the fit isn't best, or the alloy is too hard or too soft, and a few other smaller pieces of the puzzle. If I didn't know what size my gun preferred, and didn't know what alloy would likely be best for my application, and I didn't want to worry about the flat-base v. beveled-base v. gas-checked thing, I would avoid lubed bullets and buy powder-coated, then just load and shoot them.
^^^^^^^^ This ^^^^^^^ Well said! memtb
