Guys: I have a mauser sporter rifle that appears to be a guild rifle but the bore is a 30 cal by what i can see. The markings on the underside of the barrel read Top to bottom, 446 BUG 7,6mm 51 1/2 10/28 Above woodline by rear sight KMG N12gr. This rifle is on a nice Mauser 98 action with three leaf express sights and oberndorf style bolt handle. Slim stock and light. What cartridge is this chambered for as i would like to shoot it. Someone said that it was a 8.15x46R but this is no 8mm. Also the twist is about 1-9.5" Any ideas? Thanks.

7,62 x 51mm is the same thing as .308 Winchester. Perhaps it's a 308 Win. But I would have a gunsmith verify if it was chambered for the civilian 308 Winchester or the military 7,62 NATO. I understand there are some differences in the neck, at least that is what they say here in Old Europe.

Concerning the markings near the sights, 12 grams is approximately equal to 185 grains. Perhaps the sights were regulated with a bullet in that weight, or near to it.

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You need to make a chamber cast.

Alf: I did a chamber cast of this rifle and the casing it takes is quite small,like a 30-30. In fact i tried running some 30-30 ammo through the mag and i could almost close the bolt. The markings i described are top to bottom in the order i described when the muzzle is pointed to the ceiling. The markings are horizontal with the muzzle vertical if that helps.I lost the chamber cast. But the bolt face fits a 30-30 rim perfectly. 30-40 krag was a bit too tight as was 303 brit. This gun was set up for a rimmed round but what? Thanks for your help.

ALF
Maybe you do not know, but in Argentina the 7,65x53 caliber was reloaded with this weigth of 185 gr.
Several times I adquired in Argentina for reload the cartridge, FMJ bullets in 185 gr for my two Mauser in this caliber (7,65x53 Belgian / Argentine Mauser).
I know 3 military rounds (bullets weigth) for this caliber in my country: first (for the model 1891) with 211 gr cuproniquel bullet (not S) and second (for use in the 1909 model) with 173 gr and 185 gr bullets spitzer point.
At this time, I have 200 or 300 bullets in this weigth - 185 gr.
Thanks for all your precise answers in this forum, I appreciate them.
I'm sorry for my bud English.
Regards.
Hector