ok, here is one that will blow your minds. granddad found it...

ok, here is one that will blow your minds. granddad found it it a attic, 20 ga, trapdoor springfield. date on lock, 1864. date on trapdoor, 1866, with eagle head!!! fits 20 ga slells perfectly, i actually shot it with a light bp load!!!! da was pissed!!!!! WHAT THE HECK IS THIS??? WONDERED FOR YEARS!!! dad sez its a bannerman conversion. im skeptical. what about the eagle headbelow the 1866 date? HELP!!!!

It sounds like a pattern 1864 springfield rifle that was converted to breech loading in 1866. There were several different types of conversions, Allin being one of them. A good US military rifle reference should show examples of these. The converted rifes were designed to fire black powder cartridges, in either 50-70 or 45-70 calibers.
While Bannerman sold many surplus rifles, I have never heard of them being converted to 20 guage, and I would strongly urge you NOT to fire modern shells, particularly shotgun shells in this rifle until you consult a qualified gunsmith. I would expect a conversion to 20 guage would have been stamped on the barrel to differentiate it from an unaltered rifle.
The stamping of 1866 and eagle indicates a new made lock plate as apposed to a reused percussion era one. The converted Civil War era muskets were soon replaced in service with the various Trap door models by the 1870's, rendering the earlier conversions obsolete, and they were surplused.
If you want to fire this rifle safely, you may be able to buy brass cases and bullets from Dixie Gun Works, Union City TN. They sell original and reproduction black powder items. Do not use modern smokeless powder, as the pressures are too great and my damgage the rifle and injure you!
Black powder or a synthetic like Pyrodex is available, and can be safely fired in the proper case, bullet and load combination. Dixie's catalog probably has that sort of information in it.

I mis-typed part of my reply. I meant to say that the 1866 and eagle (indicating the US arsenal, Springfield) on the trapdoor is the date(or model of)that part and suggests this is one of the early conversions such as the Allin.
The 1864 dated lock plate would be the same as used on the percussion model of the Springfield rifled musket.
I have been up all night and my brain is tired!
I hope that helps.
T.