Here’s an Australian study that looked into the effects of flash suppressors on machine gun accuracy (burst dispersion). Australia uses a locally-made variation of the FN Minimi, with some local modifications including, significantly, a parallel-sided rather than tapered barrel, as the F89 in the squad automatic/LMG role. The F89 closely resembles an early (pre-Product Improvement Package) M249 SAW.
A test by the Aeronautical and Marine Research Laboratory of the Australian DOD’s Defence Science And Technology Organization looked at flash suppressors’ influence on accuracy. The test mostly looked into dispersion of rapid fire, not cold-bore accuracy, but is extremely interesting. One of the things they discovered: that an adapted FN-MAG flash-hider is superior to the standard Minimi arrangement, at least with respect to accuracy.
It has been found that the addition of a flash suppressor from a MAG 58 machine gun Will reduce the size of mean radius dispersion by as much as 41% over a Minimi barrel fitted with a standard flash suppressor, and by 35% over a bare F89 barrel.
Some part of that is due to the weight of the flash suppressor:
The evidence suggests that the parallel sided F89 barrel is stiffer than the taper ended minimi barrel and that a muzzle mass would reduce flexing in the Minimi barrel but have minimal effect in the F89 barrel.
A very interesting finding was that each flash suppressor changed the point of impact of individual rounds in the same direction and amount, regardless of the indexing of the flash suppressor.
As everyone’s come to expect from a scientific document, the scientists conclude by suggesting that what is needed is more research.