That might depend on the length of the firing pin, I think. It is possible the top end would blow out, due to the size of the hole in the nipple. but might not, due to the construction of the threaded end piece. If that end piece fit fairly tightly against the sides of the cap, it might not fragment at all & be fairly gas-tight.
English cap lock rifles are noted for this sort of feature when they have the tapered hole in the nipple, with the tight, small diameter hole at the bottom of the nipple.
The shotgun does not produce as much pressure as-does a rifle, as well.