Distance?
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Distance?
I've gotta say that given the different conditions under which these sets of groups were obtained, and other differences that you haven't even noted. And given the number of groups, I don't see a significant statistical variation here. Certainly not nearly enough to draw any sort of conclusions.
OTOH if he had done:
1) a minimum of 10-20 groups each,
2) on the same day,
3) under windless conditions,
4) using the same ammo and from the same batch,
5) with a clean rifle and barrel between sets,
6) using a bench rest,
you could then begin to draw some conclusions on variations of accuracy that a muzzle device may cause.
As it sits, the information provided is pretty close to meaningless in supporting the assertions being made.
What was the brake torqued to? This can often be the cause of such issues if over-torqueing occurs. There are times where the recommended torque may just be too much, especially if the shoulder is not cut perfectly concentrically of if there is the slightest amount of pressure on the shims that is not uniform. May not be the issue here, but possible, as these issues all also occur with bolt rifles.
This is not an isolated rifle or incident, and the same shooter over hundreds of rifles has told me that 0.25-0.75moa increase in group size is not an atypical situation when adding a brake. This is simply the first time I have personally seen it demonstrated so directly.