Tuning 600mm Barrel Kit .35 FX Impact M3

In the Impact, you will likely be able to squeeze a bit more power out of your 800mm barrel as compared to a 600. The 800mm will do this at lower reg pressures as well so a win win situation. Longer barrels are usually better on airguns in this way. 

But accuracy wise the 600 and the 800mm should both shoot pretty much the same until you reach the power threshold for the 600. After that the 800 will do better.