Accuracy starts with a precision barrel and ammo that is tuned to work with the barrel harmonics, which should also be tuned to have zero vibration at the muzzle when the bullet exits.
Military rifle slings are almost universally attached to barrel mounted anchors, because they are more durable, and military standards are 2MOA. That's typical with most AR's, as milspec barrels are button rifled oversized and chromed back to final bore size. Some makers then air gauge them and sort out the ones with more deviation than allowed, it's a matter of tens of thousandths of an inch.
Ah, no, they don't scrap the "bad" ones. They get sold, too. Very few are really dangerous, but a lot are less than prime.
Once an accurate barrel is properly mounted, ammo tested and made specifically for it, and the scope and shooter dialed in, then a free float handguard might make a difference, largely because the front sling mount will be fastened to it, not the barrel.
Technically, a shooter would be just as good with handguards attached and no sling tension at all. The difficulty is that you have to be able to discern 1/4 MOA differences, those barrels are few and far between, and so are the shooters.
The rude truth is expensive scopes, free floats, stocks, and triggers can't do jack to help the accuracy of a $200 barrel. We certainly wish they could, we'd all have cheap high precision rifles. It really takes a match grade $500 barrel, and the ammo, scope, free float, stock, and trigger to get there - if the shooter can actually shoot the guns limit, not their own. That takes years of practice.
What you read on the internet in the way of endorsement is usually justification for spending money, and setting up a heirarchy of status. "I have one, therefore I rank higher than you." It's testosterone challenged logic. None of the posters will show an empirical analysis of before and after to actually prove the point.
And it only counts on paper, anyway, 2MOA is enough to drop bullets into a 8 inch circle out to 400 yards, which is the common outer limit in shooting a live target - which has an 18" round kill zone in medium game. It's the prairie dogs and antelope that need a lot more. Not so common this side of Kansas, and about as rare as an open 500m public range.
I'm finishing an AR build, using rifle handguards. I have saved the cost of a good scope not buying an unneeded free float, and it will still do ok for hunting - that really depends on my skills, not the gun. Just spending the difference on expensive ammo alone would get me over 200 rounds, which is about ten years of hunting ammo.
It's a matter of balance, something those who drink the marketing hype are unable to achieve.
