some years ago, when "slug zone" meant 75 yard accuracy with the model 12 and a full choke duck barrel, we tried to make an improvement. The guy iw worked with had a old rolling block in some obscure BP cartridge, I think it was a 11.something egyptian, but that may havejust been one of the options, well we figured that a 20 gq was only a few thousandths bigger than the case head for that chambering. we opened the chamber to fit a 20gq and the creature would extract and so we figured ok why not.
he called around and found someone who had un turned 20 ga barrels and had one rifled. (this was before you could find them on the open market) after fitting and chambering, we loaded some 338 cal bullets in a improvised sabot, loaded up a few rounds and were extremely disappointed, They seemed more like squibs than rull power loads. we decided to try to increase the diameter of the teflon rod OD in order to get a beter burn rate after going up and gett better but still not great performance we then made one that wasa almost a crush fit into the lands and then set that off....BINGO nice big boom, little smoke not a lot of grunge left in the barrel. in the end we were getting something like 2100 fps with 225 gr 338 bullet and accurate to 2 inches at 100 yard, 4 inches at two hundred yards all of a sudden all those big field deer that had walked past us knowing we could not touch them were fair game. this gun was used for several years till he started playing with some mauser (siamese) and 28 ga cases and some smoking loads. It was great fun while it lasted.
I do know that in the 28 guage, he ended up using paper cases because surprisingly they withstood presure better. I think he was using 45 70 305 grainer bullets at 45 70 velocity out of a 28 ga casing.
That lead to some one making a barrel insert for a 20 Ga single that would allow it to shoot 45 70 rounds, that however got taken by the DNR, for violating the shottie only zone. but the barrel insert actually worked quite well. There was a savage 4tenner tube at the store and that was used as a model on a blank that was chambered for 45 70. This was all done by real gunsmiths and machinists, none of this should be done unless you KNOW you have the skill to do it. Most of the test firing was done in a vise with a sand blanket over the weapon. I am not a smith but can handle a mill and lathe and was under supervision on all that i did.