I had one of those that broke becaused it cyrstalized in my System One they are similar to a nipple. I just switched it out as I had spares.

My experience with 3f has been that you lose the expected velocity advantage when you move to the heavier bullets...at least in my guns over my chrono. My hypothesis is that the bullet takes long enough to get out of the barrel that you burn all the powder anyways.

Of course, all the barrels I have tested are 26in. I have not tested my brush guns as I do not feel the small velocity difference makes it worth screwing with a second powder (I often carry 2 guns afield) when it's a short range proposition anyways.

Breech plug design improvements have certainly made the newer designs more user friendly.

Probably not needed UC, popping the retainer spring is not uncommon thing.

The nipple itself is very strong - In this case the blow-bac pressure - pushed the primer back to the point it was able to get pregnant enough that a side blew out while still partially in the nipple and stretching the retainer spring to the point of distortion. Actually if you you took the time and had the right tools - you could re-bend the spring to the right diameter and re-install it.