Flash cups are not always the problem and often only half the solution. I am assuming everyone knows the high side of the cup goes to the rear as a barrier, but it also needs to be so high that the hammer barely clears in the down position. You need as short a nipple as you can use and the hammer recess needs to be so deep as to cover, at the very least, the cap. A tad more is better. The hammer nose inverted V opening needs to be on the generous side to allow particles to be blown forward with minimal impedement and the forward side of the cup needs to be low enough to allow the particles to clear without ricocheting back. The inverted V also needs to begin at the inner hammer face. Your mainspring needs to be strong enough to not allow the hammer to be blown upwards when you fire. Too large of a hole in the nipple can also cause the hammer to lift up on firing. Most factory produced percussion hammers do not meet the criteria I have described and need to be modified to do so. You can do this yourself with files and a Dremel type tool. You want to be sure that the inner hammer face strikes the nipple squarely for reliable ignition, so if you do this go slow and careful.
