Like I said, I have limited experience in 20 bore pre-WWII for now, although I do shoot modern Ferlachers in 20 bore. I typically see 28 grammes in 70mm lengths so if you can stuff 28 grammes in 2 1/2" and the tube steel is Krupp or one of the newer Electro variants, it will survive but you will be @ or near the max. I for sure wouldn't chance it with repurposed tubes from older platforms or pattern welded tubes.

Serbus,

Raimey
rse