Ackuric is correct in what he said! Many people here know a lot more than I do but I have tuned my Impacts probably hundreds of times for different slug weights, over and over again. Way too many times to count actually. I just wanted to share some of the things I have learned. Some call the process harmonic tuning, others call it velocity tuning but really you can't have one without the other. It's a process with necessary steps and each of these steps are parts of the whole tuning process.
What I have found and wanted to point out is that there are many ways to tune a gun depending on what you're wanting to achieve... Maximum efficiency, power, supreme accuracy, quiet, etc... Other than efficiency and power, most go hand in hand...Efficiency, accuracy, report and ES. What I'm talking about when I say there are many ways to tune your gun is this...Some may prefer to tune with more hammer spring, other's more regulator pressure and less hammer spring, some leave the power wheel on maximum, others use it for different weights of ammo or to increase-decrease velocity, some leave the valve adjuster up front wide open and don't touch it(Matt D.), others use it to decrease maximum velocity for the final fine tuning of the gun.
I like to shoot slugs and want accuracy above all else. Slugs usually like to be shot pretty fast for best accuracy(940-100fps) so I want my gun at near maximum power. I start around 135-145BAR, and max. everything else out. I get some velocity readings and check accuracy. If the velocity is in the range I'm looking for to start, I will back off the hammer spring pre load just a touch if I feel it necessary(about a quarter to one full turn) and use the valve adjuster to fine tune from there...About a quarter turn at a time until the velocity drops 10-30fps and the groups tighten up, which is usually somewhere around the 3%-5% of maximum that Ackuric is talking about. I don't mess with the power wheel, hammer spring preload or the regulator after that initial velocity and accuracy testing, I use only the valve adjustment knob up front. I want power for slugs above all else and really don't care about shot count, ES if it's under 10fps, or how loud the report may be. If I don't need maximum power for a particular weight of slug, I will turn down the regulator to start and max. everything else, as I mentioned before. Just keep in mind there are a lot of variables and different ways to accomplish the same goal. I'd recommend only changing one variable at a time and taking very good notes.
Good luck! Stoti
