My choice

M1 Garand

My preference is the M1 Garand rifle, closely followed by the Mauser.

Altough the Mauser is the best in certain categories, the M1 is the best overall rifle. The 8 rounds en-bloc clip and the firepower that comes with a semi-automatic rifle make it a winner.

Moreover the lower recoil helps long term use.

If I was to select a rifle as a base for a sniper rifle, then the Mauser would be my choice.