As I have posted before, most 450/400's were
.411 although of course with lack of standardisation
you see them from .407 - .412 but in general if measured
correctly I think you'll find most are .410/.411.
And it doesn't seem to matter whether it is 3" or 3 1/4"
althouh you should also slug the bores.

I believe It was Searcy in the US who started the .408
bore dimension with his 450/400.

In regards to barrel wear, I don't think any of us
are going to shoot our guns enough to even notice
a difference.

500 Nitro