Ditch the used brass, start with new because 9 times out of 10 the custom chamber is going to be tighter than factory. You said they chamber & extract fine before firing, but I still would use new brass with a new chamber.
Buy a chronograph, a prochrono digital is around $100. It'll tell you right off the bat if you're too heavy on the charge weight. I won't fire the first round through a new barrel without the chronograph out, it's a waste of components in my opinion.
