You won't be sorry in the long run. If by any chance one splits don firing, maybe you stop and re-examine. I once ruin a box of 577/450. It was lathe turned and I did not notice the splits until the box was about shot up. It should not be a problem for you, but look over the first couple fired cases well in detail. You were not specific in your bullet description, just an old Lyman. They made several. My point is, cast in pure lead they will be smaller in diameter, about 0.001". That may help you. I suggest you go slow when loading/seating. Sounds like you have a minimum size chamber. Maybe a little flare to seat bullet, then a little full sizing to take out the flare so they chamber. keep us posted and good luck