I know there are those among us who will declare that pinching a cap is done all of the time and it is a safe and accepted practice which does not promote chain fires. But as the bullet fit well and the chambers were aggressively schmutzed I have to believe that the gases came from the aft end of the cylinder. I don't pinch caps any more.
The one I have has #9 nipples. All I could get back in the day was #11 but like I said I can leave all the caps completely off of it and as long as the balls are lubed it will not chain.