Well as for the chamber depth issue, I guarantee the .454 factory barrels are chambered plenty deep for a 2 1/2" .410 shell. I'm amazed at how long the throat is on my barrel, I can't put a 405 gr. cast bullet into the belled mouth of the case and touch the rifling, that's how long the throat is! Now that my curiosity is peaked, I'll have to put a depth micrometer to it and see exactly how long it is, maybe it's not long enough for a 2 1/2" shell, I may have been shooting my mouth off back there, hmm, that will occupy for a little while today...
Selmer