My factory new FX ImpactX in .22 had the reg set around 105, and the hammer spring slider is around 18.5 mm's. The velocity was "near" 880 fps using JSB 18.13's. That's how they come. Many shooters want more, so they tune for speed using a heavy for caliber pellet/slug.
When you say you needed to added A LOT of hammer spring adjustment, that's kind of a red flag if it was my new Impact in .22. From the factory _in .22_ when you turn the power wheel to MAX, you can still wiggle the hammer spring slider. AND... you still have a range of velocities to use. Mine went from around 680 on _MIN, 1 and 2_, to around 880 on MAX with about 50 fps difference per number from 2 to MAX. I did up my reg to 110 and am getting around 900 fps with the JSB 18.13's. It's accurate out to the limit of my 90 yard range.
Maxing out your hammer spring will likely be a little tough on the C3 bumper. When that fails (and it will) you will loose velocity, and/or have varying velocities, mostly low. I was told from the tech at FX USA that the reason FX Sweden (where the guns are made) do not use the delrin C3 bumper is because you loose some of the ability to vary the power with the power wheel. To some that's not an issue, there is only one setting, and it can last a lot longer than the factory O-ring.
