I had some work done to my CYMA/ Colt M4 CQB-R... Even before the upgrades, it would shoot straight, but hard left (as if I was aiming THAT way) . I put in a new buckling, tight bore barrel, piston, spring, spring guide with bearing, and motor... OEM parts chrono'd it at 285. Uprade chrono'd to 395... Both times with 0.20 BB's. And Yes, I did adjust the hop-up, OEM and Upgrade.
It's still shooting straight and left, but not that far left as before... My sights are maxed out and still beyond limits.... I rolled the barrel and it's straight as can be..