i know this has been asked quite a few time but i cant seem to get a straight answer

does a flash suppressors affect accuracy of a rifle?
do some affect accuracy and some dont?
yes or no
then explaination plz

to a minute degree yes.
Take a perfect barrel.  When you fire a bullet down it the barrel vibrates and twists counter to the rotation of the bullet like a water hose that has pressure suddenly applied to it.  the harmonics are closer to being the exactly the same because the barrel is a perfect homogenous tube.  Now add a threads and piece of metal to the end and you loose the harmonic movement.  NOW add a hunk of steel halfway down the end of the barrel and stick a tube on it that leads to the bolt.  Every time the bolt closes the bolt whacks the tube and the barrel vibrates.  

This is all super technical wonky crap that really does not matter to most of us.
So no it doesn’t make a difference at 3/4 MOA or greater.

 To the extent that the FS protects the rifling at the muzzle from damage, it may actually help preserve accuracy.  I think that everyone agrees that the crowned "plain" muzzle is the most inherently accurate, but for practical purposes I see no reason that a FS would hurt accuracy.  Practically speaking, it's good to have- not throwing a huge fireball is also conducive to better concentration on your target.      

btt

Quoted: btt

They pretty much summed it up. Not too much else to say about it.

It is said that the Army Marksmanship Unit tested the effect of flash hiders on accuracy, and concluded that they could degrade accuracy significantly if the FH was screwed on tightly. They supposedly found the best accuracy when the FH was only screwed on hand tight.

A few months ago, I tried (unsuccessfully) to find the original sourse of the above. It sounds plausible to me, but I can't be sure until I see the original AMU report or other reputable source. Just the same, to be on the safe side, I did install the muzzle brake on my IPSC rifle using a lock washer  that I did not allow to bottom out (rather than a peel/crush washer), and I locked in the index using red Loctite.

Perhaps this is a test that someone here with a steadier hand than I might like to perform... how does FH torque affect group size ?