After initial research I have understood following:
Long barrel does not give sniper accuracy. Barrel back gives the speed and the shape of barrel front forms the signature of a shot.
Curve ball will be less affected by wind and other elements.
Apex, flatline and rifled barrels gives such curve. Apex uses a short contact rubber to spin the paint, Flatline uses a long friction surface in a curved pipe to the same. Some pipes uses twisted rifles, ”all american” fronts has twisted venting holes.
Venting holes reduces the sound.
The back diameter is claimed to determine accuracy of the ball. If the diameter is too wide a lot of air misdirects the ball to bounce on the walls as a snug fit will push the ball evenly. Overbore and Underbore explained with eggs
However the atomic structure of gelatine shell is built completely different than naturally produced egg shell. He speaks a lot of statistics and in a scientific manner but I tend to agree with him. It is not the barrel size what is significant in ball brake but the feeding of balls is. Below visualisation how balls brake is generated.
The ball diameter is never exactly identical and every so often one ball could bee larger than the average size variation. Even if the balls are stored tempered, during the game they are not. So at the beginning of a game the shots could be consistent as during the day balls deform the consistency begins to suffer. I guess high quality paint has better skin that does not deform as much and also the size variation between balls is more consistent giving more accuracy.
More Science behind the barrels by Inception Design
If purchasing a marker make sure the stock barrel is good enough or the prize of the marker is low enough so you are able to purchase insert barrel. As stock barrels Boomsticks and Shaft barrels are good enough. Tippmann barrels need to be upgraded. I recommend to purchase a freak kit so you can underbore and improve accuracy.
How to bore match your paint balls to your barrel.
Unscientific way of comparing barrel kits
Favourite PB Vlogger describing paint in barrel
Ball Bust in slowmo – commented