Re Mr. M Grubers earlier comments.
Yes your analysis would be correct but not all encompassing.
If the chamber were let out here in the US it would almost certainly have been done with a commercially available reamer ground to SAAMI spec of .797" thus your 800" would still stop at 2&1/2 ". you would need to visually check for a very minor step.
I find it useful to have a plug at .796" to double check for this condition.

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745