Re: How much bullet into the case neck?
My 22-250 is likewise built on a '98 Mauser with a 12 twist Shilen barrel. I am mystified that you are having trouble with the 50 and 55 grain bullets. When I had a 14 twist A&B barrel, the 50 VMAX and 52 AMAX shot the best. With a 12 twist, the 55's are shooting VERY nicely and I am about to try the 53 VMAX.
At any rate, I have found that many 22-250 chambers have a lot of freebore. My advice is to not obsess over getting close to the rifling. When I actually test for seating depth, I have found that almost every rifle I own shoots more accurately with significant jump to the rifling. My advice is to start with your bullet seated as far out as you are comfortable seating it, then do the Berger Bullets seating depth test. It doesn't matter what bullet you are using. Your rifle will more than likely tell you what seating depth it likes with a given bullet. After you figure that out, do your powder charge run up.