Most benchrest shooters would tell you not to neck-size down to the shoulder as you're going to get inconsistent 'grip', as you put it, around the junction. Tony Boyer (12-time World Champion, 11-time Shooter of the Year) writes about just this exact issue in his book "The Book of Rifle Accuracy" - you should definitely pick it up, it's not just for Benchrest shooters, I learned a lot from it. Provided that you've turned your brass to a consistent diameter, he only sizes his necks down approximately 2/3's of the way,... eliminating the inconsistencies of the neck/shoulder junction. I'd suggest that you take some measurements with your fire-formed brass and order up another (smaller) bushing to get the amount of tension you're looking for. .002-.003 tension should be PLENTY to hold it stable.