On average I believe a bolt action MZ will shoot better than a break action MZ. Sure there will be some that wil overlap. But in general I believe the bolt action will do slightly better. Same as with Center Fire rounds. The bolt there will also be a bit more accurate than the break action. It is simple. The bolt action is stronger. That is not knocking a break action, but it is the way it is.
I really like my Encore CF's. But none of them(including custom barreled ones) are as accurate as my bolt actions in general.
I have seen many people have problems getting a break action MZ to shoot accurately. I have never seeen any accuracy problems with bolt action MZ's. But then I have seen limited amounts of bolt action MZ's. Most of the ones I have seen have been Knight's. And most Knight's just flat out shoot. But I have also shot some of the Remington and even the Ruger bolt action MZ's, and the few of those I have experience with are also pretty good shooters.
One last thing. I have been shooting MZ's for over 40 years. To this date I have never seen any MZ of any form shoot a one hole group as stated above. Tom.