ORIGINAL: Pygmy
I'd only be guessing without seeing the picture, but it sounds like one of the Russian or Finnish Moisin Nagant variations...
Are you sure about being ISSUED to your grandfather..??.. I know of no straight bolt rifles issued to US troops...
After the collapse of the Czarist regime in 1918, the US gov't seized a great number of M/N rifles that were made here (Rem. & N.E. Westinghouse), and used them for training rifles because the M1903's and M1917's were in short supply.
In addition, the 27th U.S. Infantry (Wolfhounds) Regiment thatwent to Vladivostok, Siberia to fight the Reds were issuedM/N'sinstead of 03's due to ammo supply problems.......
http://www.kolchak.org/History/Siberia/aef.htm