The caliber of the drilling is 6.5x58R Sauer & Sohn. This cartridge is also known as 6.5x58 1/2R ( 8x58R and 9x58R are both some times shown as "58 1/2"- case length is actually between 58mm and 59 mm).This cartridge was a popular cartridge,especially for hunting where the largest game expected was Reh (Roe)deer.The 25-35 Win. aka 6.5x52R is a distinctly different cartridge with similar ballistics,and was used for the same game,but very different case shape.I never tried,so I don't know if a 25-35W will even chamber in a 6.5x58R.Cases can be made from original length 38-55 cases or bought from Buffalo Arms or Bertram at a higher cost. Bullet diameter is usually.260-.262" rather than the normal .264" of 6.5mm bullets. The normal 6,5mm bullets can be resized to the required diameter,or some people have satisfactory results with .257" 25 cal bullets.If you just will not handload,there are custom handloaders that will do it for you. Your local gunsmith will likely help you find one.
The diameter stamped on the barrel is the bore,not groove or bullet diameter,and the length is the length of the case.Neither is necessarially the same as the nominal caliber designation.These marks were placed by the proofhouse,not the manufacturer.
I hope this helps-have fun.BTY the 16 ga. chamber will be 2 9/16",not 2 3/4".
Mike
Last edited by Der Ami; 01/06/13 12:58 PM.