Flattened primers may or may not mean something, particularly with regard to handgun loads. It sounds as if the only thing you changed was the coated bullet for the uncoated one and primers, powder, and cases stayed the same. Is that right? What was the diameter of the uncoated bullet and the coated one? Be sure you measure with a micrometer and not a caliper. If you were getting flattened primers with the only change being a coated bullet, you might have high, though not necessarily dangerous pressure. Was there something wrong with these bullets before you coated them (poor accuracy, leading, etc.)?