I have become a "hoarder" of late when I find something I like. Those bullets look a lot like a lengthened Ball-et I have tried before.
I had a gentleman cast some Lee REAL's for me and they shoot well, problem is they are so undersized that I can literally shake them out of the bore.
Have you tried giving them a good "rap" with ramrod to partially obturate them after seating?
I like REAL bullets and use them in my two Thompson New Englanders (1:48) for plinking because they are easy to load. I certainly can't shake them out of the barrel, but for plinking, I like 60 grains of FFFg, which obturates them well. Light loads with FFg doesn't seem to obturate them well (poor accuracy). I killed an antelope with a 300 grain 54 cal Real bullet about 25 years ago. It was my "flat-shootin'" bullet way back when.
They probably need a good jolt to "bump" them up, as they are flat based and light.
If you require a larger diameter, you might want to ignore the Buffalo Bullets too, as they are fairly easy to load in my New Englanders(so may be undersize for your bore). However, the Hornady Great Plains are hard as H to load and the TC maxi-hunters are just a bit less so. I like the looks of the Great Plains, but they are far too tight for a quick second shot. I literally had to push them in by shoving the ramrod against a tree and pushing the rifle into the tree to get them seated. Hornady used to sell flat-nosed Great Plains, but I think they now only make HP, except the 50 cal Pennsylvania bullet. You could check Graff & Sons for any discontinued bullets as they seem to have stores of older bullets. They have the lighter, 350 grain maxi-hunter listed as well, which is fairly short. The Great plains Pennsylvania bullet is only like 240 grains, so need to get close. Yet a 50 cal round ball at about 180 grains, or a 54 ball at about 220 grains will kill elk.
If you stay with modest loads of BP, and not the new super loads of BP subs, a Minie bullet works well. I use a cheap Lee Minie mold to cast .533-inch dia. 410 grainers. They shoot well in my 1:48 twist.
This photo includes a 300 grain 54 cal REAL bullet (center), and on the far right is a 50 cal HPHB Buffalo Bullet that I sent a bit faster into the medium. Its kinda cute; got a hole right through it; like a donut.