Because it says "S" it probalby has a .323" groove diameter.

It is easy to make an error of .002" when trying to measure the groove diameter from the muzzle, with calipers. oneed to rock the tines back and forth while pulling back on the button, watching for the largest measurement. Even then, mistakes are easily made. Slugging the muzzle with a pure lead slug and measuring the slug is the most accurate method.

That the .223" bullets fit in fired brass is the biggie here.

The age of the gun does not make it too weak for normal pressures, which is what Norma ammo is loaded to - providing the front receiver ring is not cut almost to the threads with a dovetail. Of those rifles, I am leary. This rifle appears to have the front ring dovetailed into something attached to the front ring.

Phil's spot-on - that is a nice looking Mauser.