Myself and another member built custom 410 fully rifled deer guns for use in Ohio about 4 years ago before they legalized some strait walled rifle cartridges for deer gun season.
We were shooting heavyweight 44 cal. bullets out of plastic cased 410 hulls and found the 209 primers to be the weak link as to how far we could go with pressure before something failed.
In Ohio if using a shotgun you cannot use a all metallic case so we found WW AA plastic casings to take quite a bit a pressure ( 1 time ) but at a point probably above 25000 psi the primers would come apart but not with any danger to the shooter using a H&R handi rifle that fully supports the cartridge.
With a 20 ga. using saboted bullets with a higher BC at similar pressures should be a very effective deer gun.
Jedman