I have been seeing many piston designs with gas off settings. As far as I know, this setting simply blocks the gas from traveling into the gas block, making it a bolt action rifle.

So does channeling all the gas through the barrel actually affect the muzzle velocity of the projectile??

Some people are saying it has a negligible effect. What's the truth?

Does anybody have a 556 upper or a 762 39 to confirm this?

Yes there would be a MV difference and Yes, it is negligible depending on why you are asking.

You can simply deduce this difference by looking at load data and factoring in barrel lengths for the same load between .223 Rem AR-15 and Bolt Action rifles.
Sierra Edition V, testes the AR 15 with a 20" barrel and the Bolt Action with a 24". 69 gn HPBT MK diff with H335 at 2700 fps is 0.8 gn which would require even
less of a charge in the AR to achieve the same MV given the 4" barrel difference.
Or
You could always grab a chrono and shoot ten of each, grab your mean and have a more accurate real world number.  

I wouldn't waste the time or ammo unless I was trying to work a load for a specific situation.
Are you?