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If your not 100% sure it's chambered for 30-40 and you're not comfortable doing a chamber cast, I'm sure a local gunsmith would do it for a nominal fee. 

Just because a barrel is marked one thing doesn't guaretee that it hasn't been improved or completely rechambered to a case size larger than what is marked. 

I own a very nice sporterized Mauser that was tagged as 8mm on the gun sore rack that turned out to be 30-06.  Over the decades I've read many times of folks finding surprises, sometimes the hard way at the range, sometimes with nasty results.

I'm a Mosin fan myself, and your Granfathers rifle is a very interesting piece of history!