1903 Sporter in 375 Mag

I recently purchased a 1903 sporter rifle that is chambered in some 375 Mag. Initially the rifle was sold as being chambered in 375 H&H Magnum (which was a fair assumption given the rifle's age) but I tried to run some 375 H&H Mag snap caps through the rifle, but had trouble fully closing the bolt on the round. I don't yet have any 375 H&H Mag ammo to try in the rifle (to verify that the snap caps aren't ill sized) but I'm wondering if it might possibly be a different 375 Magnum round than the H&H.

My first guess would be 375 Weatherby Magnum (although I thought you should be able to chamber a 375 H&H in a 375 weatherby) but does anyone know a similar round which could be the possible caliber? If the rifle really is chambered in 375 Weatherby, is it possible to have it re-chambered to 375 H&H Mag (which is much more commonly available and cheaper) or would it require a rebarrel?

Here's a picture of the action for those of you who enjoy some big bore, safari-style sporters!

The bolt face seems to fit the belted mag well and is too large for a .30-06 case. Strangely, the snap caps seem to feed perfectly and will actually extract and eject. The bolt just barely won't close (it seems to rotate about 10-15 degrees instead of the full 90 degree for full closure)...in fact it seems to be almost fully close but I simply can't get the bolt handle to fully rotate.

Was 375 Winchester ever referred to as 375 Magnum (the barrel is stamped 375 Magnum)? I'm thinking that a chamber cast will probably be the only way to know for sure.

It seems like the action is set up perfectly for 375 H&H as the snap caps feed perfectly in and out of the camber (excepting the last little bit of lock up). You can see from the below picture that the controlled feed is working perfectly.


Here's how the rifle looks closed on an empty chamber:


And here is it at the maximum I can close the bolt with a snap cap in:


It doesn't seem like the bolt really has any more room to close towards the breech, but it simply won't rotate all the way down to fully closed.

I guess at this point, the only thing I can think is that it must be some sort of wildcat based on the 375 H&H. I'll probably have to a chamber casting to figure out what it is for sure. If it turns out to be a wildcat, I'll probably try to look into having it re-chambered for either Weatherby or H&H. Does anyone know of a good place to have that done or can most gunsmiths accomplish such a job?

Thanks for all the replies, gents. It's a quandry. I may just go ahead and by some live 375 H&H and see how it works out.

Jim, here's a picture of the barrel markings. It's fairly vague but the makers were apparently popular in northern CA in the 50s/60s.