The threads in the breech for the nipples on my shotgun project appear good in one, not so good in the other. They are 1/4 x 28 now. I want to go to a larger size threads but I'll be damned if I use metric threads. It appears the next choice up then is 5/16 and musket caps. I have no problem with using musket caps and I suppose there is enough material in the breech to handle the larger holes.