To get the best accuracy, you don't want anything touching the barrel if you can avoid it, that's why match ARs all have floated barrels - so the handguards and sling pressure are not on the barrel.
That's why in (I think I have the right movie) The Band of Brothers, there's a new recruit fixing his bayonet on a Garand, and the sargeant tells him to take it off because the rifle won't shoot worth a darn with it on.
Accuracy is affected by barrel dynamics, where the pressure of the powder causes teh barrel to swell in length and diameter behind the bullet, as the bullet travels down the bore. All of this causes the barrel to vibrate, and adding mass, especially non-symetrical mass, will affect where the tip of the barrel is pointing at the moment the bullet leaves the muzzle.
If you walk up and down the line at the National Matches at Camp Perry, there is a reason why you will not see any of the new piston ARs.
Yes, more weight helps stabilize a rifle during aiming, but in a match rifle, those weights are placed in a shroud that surrounds the barrel, so nothing touches.