Quite often what you described can be caused by one of three things. Gun related, gunk aroung the firing pin slowing it down. Often, it will fire the second time, firing pin strikes usually look very light.
The next are reloading problems, as was mentioned, when you see the gun go snap, the first time, then boom the second, and this happens several times, I suspect two things. One, the primer pockets were not cleaned or two, the primers were not seated all the way into the bottom of the primer pocket. The first hammer strike seats the primer all the way. If you look at the primer after the first strike, it will appear light. Then try it again and it goes boom, the primer strike will look normal. The build up of crud in a dirty primer pocket cushions the first blow, but is compacted by it so that it will fire the second time. Either condition gives exactly the same indications. If you do not clean primer pockets, start. If you seat primers with the press mounted arm, you might want to get a hand primer or a bench mounted priming tool so that you can feel the primers contact the bottom of the pocket. I bet that cures the problem. Good luck.
The next are reloading problems, as was mentioned, when you see the gun go snap, the first time, then boom the second, and this happens several times, I suspect two things. One, the primer pockets were not cleaned or two, the primers were not seated all the way into the bottom of the primer pocket. The first hammer strike seats the primer all the way. If you look at the primer after the first strike, it will appear light. Then try it again and it goes boom, the primer strike will look normal. The build up of crud in a dirty primer pocket cushions the first blow, but is compacted by it so that it will fire the second time. Either condition gives exactly the same indications. If you do not clean primer pockets, start. If you seat primers with the press mounted arm, you might want to get a hand primer or a bench mounted priming tool so that you can feel the primers contact the bottom of the pocket. I bet that cures the problem. Good luck.
