1916 Spanish Mauser in 7mm, or....

[SKS Ray]

I picked up a 1916 Spanish Mauser today. These rifles are pretty foreign to me with an 1891 Argentine being the only other type in this Mauser genre that I own. The rifle is in great condition overall, I just need a safety, butt plate, and sling loop, all of which I can find for about $10 which makes the total purchase only $110.
Only question I have is how can I find out for sure what caliber it is? I know a lot of these were converted to 308. Those usually have the Spanish Civil Guard crest on the receiver. This one has a semi scrubbed receiver. Heres a shot:

The only other marking I can find is 7mm stamped on the side of the barrel.

Now someone on another forum said if it was rechambered for 308, it should swallow a 7x57 Mauser bullet at the muzzle. They said if the bullet goes 3/4 or so of the way in the muzzle, it should still be 7mm. I tried this and the 7x57 bullet goes 3/4 in at the muzzle, but I don't like the word "should" when it comes to guns and safety.
Without having a smith do a chamber casting, anyone have any input on this?

 

[jhrosier]

If the 7mm bullet stops before the case neck touches the muzzle, the bore is much smaller than .308. Also the 7x57mm cartridge is about 3/16" longer to the shoulder and will not chamber in a .308 chamber without a great deal of force (mallet to the bolt handle force). While a headspace gauge and / or chamber cast would remove all doubt, the barrel marking and fit of the 7mm bullet would be good enough for me.
You should probably consider buying an inexpensive dial caliper. They can be found for around $20. The inexpensive ones are still accurate enough to measure the bore and / or land diameter of a barrel +/- 0.001".
If you don't already have a copy of "Cartridges of the World", it's time.

 

[firtree]

You can also find a lead sinker that is a little bit more than bore
diameter and drive it through the barrel. Once it pops out you can
measure the land and groove diameters, essentially a poor
man's slugging.

F

 

[Nashmack]

Whatever you do make sure you have the headspace checked.

 

[highlander]

Brownells has Cerrsafe it is a low melting lead alloy (150-170 degrees)that melts in boiling water plug the bore with a tight fitting patch pour in cerro and let cool compare casting to either a 308 or7x57 case