There are two ways a flash suppressor is going to affect the accuracy of a rifle: if the muzzle blast is, in any way, restricted or redirected; it can cause an increase in group size. Best scenario would be a nice 11 degree crown with no flash suppressor. But the gun may shoot equally well with the flash suppressor. I would also expect the old AR-16/M-16 flash suppressor with slots all the way around to shoot a little better than the new ones that have a closed bottom.

The other way the flash suppressor will affect your barrel is the way it alters barrel vibrations. In this case, the impact could be good, bad, or indifferent. Kind of like the fine tuning you can do with the Browning BOSS system. Except it's not adjustable - unless you cut a little at a time off the barrel with a lathe.

I know some shooters who are very particular about accuracy with the AR-15 rifle. DCM competition. They recommend installing a flash suppressor with very little torque - finger tight only - using red loctite to keep it from loosening.