When you buy your rifle nipples, you want the smallest possible flash hole possible in them. First because the cap will fire through the smallest hole and ignite your charge if the flash hole system is properly maintained, and secondly so that the chamber pressure will be better contained by the smallest hole, thus more consistency and accuracy. Hammer blow back is an accuracy and a safety problem. I like those vendors who have a quantity if nipples to select from. Pick up 2 or 6 and look at the flash holes closely and choose wisely. Later get a stranded wire and find a wire strand to use as a hole gauge or use a very small drill.
Eventually as you shoot you will find accuracy dropping off because of nipple flashhole enlargement. That changes the fire in, pressure out consistency of your load. Replace the nipple and go back to your good accuracy. Use gauges to figure out the nipple replacement point. Don't wait till the hammer comes back or your grouping goes away.
Lon