Say there is a 1/4" diameter space under the nipple. The thrust on the will be the same as if it were punched through to the bore. That is how we normally do it, with a drum or a breech plug. You cannot make the nipple seat against an internal flat shoulder with a pin hole communicating the flash, not easily anyway. Say you have a 1/8" fire channel and a gap under the nipple. Given sufficient pressure dwell time, the area under the nipple will receive full pressure and full area to press on. It is no different than drilling through to the bore.
Or maybe I am full of it. IF someone really knows please educate me.
Additional thought added later: How is a nipple different than a flint vent line in this respect? We don't worry about flint liners blowing out, why is a nipple an issue. Both are fully exposed to the main charge.