I have neck sized cartridges for bolt action rifles and low pressure single shot rifles but had chambering issues with double rifles and those singleshot rifles firing high pressure loads.
earlyriser the OP is reloading for a Ruger M77 Mk II in 6 m/m Remington. That's a bolt action, so he shouldn't have any problems with chambering neck sized shells that had already been fire formed to his chamber.
The chambering issues you had in your double rifles and single shots most likely stems from the fact that they don't have the camming force a bolt action rifle does when chambering a round.
In situations like that, sometimes you can get some of the benefits of neck sizing by adjusting full length sizing dies to just short of kissing the shoulder, so there is no shoulder set-back, and are only partially sizing the case body. It's commonly advised to always full length resize cases for dangerous game hunting, because you don't want any chambering issues when a big grizzly bear is rushing to feed on you.
