Tac,
Thanks for the reply and my apologies for confusing you!!!
I should have said weight rather than size. The 510 and 566 refer to grains rather thou's/inches.

The only ones I have found to date were from Kranks ref USA 523-577 with a stated weight of 620gr so a bit heavier than the more normal 535gr.

I appreciate that the minie bullet should be a close fit in the barrel and that the gasses should "seal" the bullet in the bore to ensure the rifling functions correctly and spins the bullet. I have tried one of the 620gr bullets in the barrel and I think it will fit quite snuggly.

Again apologies for the confusion.

Lansdown (Bob)