3DogMike makes a good point. My own personal experiences tell me that if a DG rifle cannot be kept at the shoulder, mounted, while cycling the bolt, you are much disadvantaged in time taken when quick follow-up shots are required. I've seen more than one hunter drop a rifle to reload then re-mount the rifle to shoot again, when the pressure is on. Good luck with that form during a charge.

A cocking-piece peep is fine on a rifle, so long as a fresh cartridge can be chambered easily with the rifle shouldered. Cheek-weld should be maintained while the action is cycled. Going to the trouble of making a pattern stock, that can be used as a try-stock on the rifle to thrash issues like this out before making the stock proper, can prove a nifty thing on a number of levels but few people go to the bother of taking that time.

The other aspect to them, is that on say an M98 action, the rear of the bolt can have a lot of slop both up/down and sideways, which varies from action to action. This not only affects trigger feel/function (variable sear positions), but with a peep affects repeatability of the rear sight. Sadly, this is almost never taken care of when mounting a cocking-piece peep, and accuracy can suffer. The peep to me is useless without proper sear mods ($$$$$$$$$ and time). Would you shoot a rifle with a vee rearsight flopping around in the dovetail? What is more, if the action is a standard one to be opened up at the rear, the bolt slop only ever increases. Re gun fit, this can be so significant as to force the comb height of the stock to be lower than it otherwise ought to have been. An action for fitting a bolt mounted peep to ought to be carefully chosen and modified.

Lock time seems somewhat slower, too, not such an issue on a big bore like a .500J. Ideally you need the long cocking piece on a 98.

Done right, they can be a great thing. Obviously, with a thin ghost-ring peep rather than a diopter they are a really fast sight.