The original bullets were .411 although I have seen
English DR's with bores from .407 - .412 but the
standard seems to be .411.
I have measured quite a few and it can be hard to
get the correct measurement. If you shoot a new
gun always measure bore first.

The .408 cam about from the US market asking for
them - I think Searcy's bores are .408 in his 450/400's.

Hope this helps.

500 Nitro