NitroX,
It depends on how you define nitro express.It would have to be a British cartridge or adaptation of an existing non-British cartridge. If you mean an updated black powder express you are ruling out all the rounds that were never loaded with black powder. This would greatly reduce the possible answers. Also, do you include belted and rimless cartridges? Big bore only, or all bores?

Assuming that you are including all the above, the most common could be the 275 Rigby Rimless Nitro Express (AKA 7x57 Mauser). Another possibility is the 303 British. In larger bores the obvious choice would be the 375 H&H Belted Rimless Nitro Express.

If you refer to only large bore rimmed cartridges my guess would be the 470 NE (if you include modern firearms), and possibly 450/400 NE (3 or 3.25) or 450x3.25 NE.

The cals that I personally own and use include 256 Mannlicher, 275 Rigby, 280 Ross Rimless Nitro Express , 300 Belted Rimless Magnum (H&H) NE, 303 British, 333 Jeffery Rimless NE, 350 Rigby Magnum Rimless NE, 375x2.25 Rimless NE, 375x2.5 Flanged NE, 375 H&H Magnum Belted Rimless NE, 450/400 3" Flanged NE, 416 Rigby Rimless NE, 404 Jeffery Rimless NE, 450 3.25" Flanged NE , 470 Flanged NE (actually got rid of my 470 recently).

Of the above, my most used cals are 375 H&H (both vintage and modern rifles), 375x2.5 Flanged, 280 Ross, 303, and 450/400 3" (in that approximate order). I have vintage rifles in all these cals that are still extremely accurate, and as useful today as when they were produced. There is just something a little more satisfying about hunting with one of these old rifles than one of my modern scope-sighted rifles!!!!

Very interesting post

. How about some photos of your rifles guys?

Jim

edited: I guess that I should have limited the choices to double rifle calibers (dumb me

). That being the case, I use 375x2.5 Flanged NE, 450/400 3" NE, 450x3.25 Nitro for BPE, and 9.3x80R (not a NE, but in the same category).