rctsat,
There are several cartridges based on the 11.15x60R(43 Mauser)case, which has the Mauser "A base" or "MB". This one has a rim dia.of .585-.595" and a head dia. of .510-.516". There are some cases even larger, but you described the rifle as a Schuetzen type,and they would be seldom, if ever, found in such a rifle.You didn't say,but your dascription seems German. If it is Swiss, there is an outside chance it could be based on the 10.3x60R Swiss case.To get a positive answer, you will need to mace or have made a chamber cast and measure it using dimensions to the nearest thousanths of an inch. you should also "slug" the barrel and post photos, including of the proofmarks, if any.
Mike