Trapdoor Rifles

Last February I picked up a very decent trapdoor cadet rifle w/bayonet for $700. Perfect bore. No pitting on metal exterior at all. Anyhow It does not seem to like 500 grain bullets at all. Does aok with 405 gn bullets.
Am using trail boss for fuel to base of bullet on either weights. Anyone really play with a cadet for shooting ?. I had one 35 years ago and shot it with BP and paper wads behind the bullet...did really good three shot groups at 100 with 405 bullets but at four or five the group always spread and I had to swab the bore to get "back in there".

Rifling twist never changed, but bore diameter vs. bullet alloy could be in play. Also, 500gr. with full charge would be brutal in the lighter Cadet. could you be flinching a bit?

No flinching , I'm kinda a recoil junkie. I size both weight bullets the same .458.

I neglected to ask if you were getting keyholes? Trapdoors tend to run oversize, as high as .462" - .463" in the grooves, and like a fairly soft alloy. Have you slugged your bore? If yours IS on the bigger side and you are shooting "hard" (less than 20-1) .458"s, that could be part of the problem.

Here’s a pic of my ‘73 cadet and an old target:
Not too shabby, me thinks (for 82-yr.-old eyeballs!

Not too shabby for sure!