You really should slug it to get the most accuracy:
"Groove diameters vary and Russian/Soviet rifles tend to run larger than the nominal size of
.311" to as much as .314".
Slugging the bore is the only way to be sure of the exact measurement."
"A bullet larger than the groove diameter can cause dangerously high pressure when the rifle is fired. A bullet significantly smaller than the groove diameter might not stabilize properly and be inaccurate. Ideally the bullet should be slightly smaller than the groove diameter. Because of the wide variation in groove diameter between models and even within models the only way to be sure is to slug the "bore", or inside of the barrel."