The Great Plains Rifle is a fine one. I had a .54 for years and I sold it in June to a friend that really wanted it.
You are right about the butt plate.
I only used roundball in mine. It had a good kick with 120 grains of Elephant FF and a 220 grain ball.
The only thing I do not like about the Lyman is that dang front sight. It looks like it is an inch thick.
I replaced it with one about .60 and it was much better.
The Lyman is a quality rifle.