I am confident that the Lancaster oval bore rifles generally used paper patched bullets. I believe you are worried about a non-issue. Your more troublesome question is what weight bullet to use to achieve regulation.

If you have a smooth sided bullet of 20:1 alloy and of a diameter equal to or only slightly less (a thousandth or so) than the "land" diameter of your rifle - the smallest dimension an inch or so ahead of the chamber, this should work correctly. Then you experiment with paper thicknesses, starting with onion skin, and working with thicker papers (if necessary) until you get a perfect seal of the gases. You can determine this by inspection of the patch fragments found ahead of your shooting bench.

This process sorts out your bullet diameter and patch thickness. Beyond that, your loading questions will be what weight bullet and what sort of wad column to use.

Powder is an important consideration. Many shooters are finding success using GEOX Olde Eynsford 1-1/2 F in black powder cartridge rifles. It seems to approximate Curtis & Harvey's #6. Our member, Mike Rowe, is having perfect success loading his Watson .450 3-1/4" BPE double with this powder and PP bullets. Perhaps he will chime in. He wins matches with his rifle and loads. I know, as I have competed against him.

A grease groove bullet might ultimately work in your Lancaster, but it would not be my first choice.

I'll say one last thing about your barrel taper. There's nothing unusual about that. My Henry rifles all have taper; at least as much as yours. There's no question they were intended for smooth sided paper patched bullets, as I have the original moulds, numbered to the rifles. The bullet and patch get a grip to start with, making a perfect gas seal just as they enter the barrel, and they maintain a tight grip and seal all the way down the barrel. Don't worry about the taper. My damascus barrels aren't troubled by it.

And by the way, welcome to the NE forums!

Curl