Ran out of characters.  Here is more.  Now you should be able to slug the barrel and get the groove diameter.  Run a couple of oily patches through the bore.  The lead slug needs some lubricant. The slug must be big enough to completely fill the grooves, which will (with luck) measure about 0.264". 

When I slug an old military rifle in6.5 or 7 mm, I start with a .36 caliber lead ball for a percussion revolver.  Oil the ball and start it in the muzzle.  I use a small brass hammer and tap the ball gently until it enters the muzzle.  Next, I use a piece of 1/4-inch brass rod stock about 4" long to push the slug a bit further into the barrel.  An old but sturdy cleaning rod should push the slug the rest of the way with little effort.  Push easily and note any places where the slug seems tight or loose.  Put a rag into the breech to catch the slug.  The slug should tell you how worn the barrel really is.

When you finish, you might use some minimally abrasive bore paste to smooth things up a bit.

My experience with old 6.5 & 7 mm military rifles is that they have quick twists and usually shoot jacketed bullets a lot better than cast.