Impact MkII, .22, 700 mm, Power Plenum, purchase new in May 2020. Having failed to make slugs work with slug liner A, I changed back to the original pellet liner supplied with the rifle. I was getting good performance with JSB 18 grain pellets, but on a whim decided to try the H&N .218 23 grain slugs which had shown some promise with the slug liner, although not good enough. To my surprise I found that with power wheel on 5, valve adjuster about the third line, regulator about 110 bar-115 bar, and the hammer tension screw about halfway in (I found I had too much variability in measuring the length of the slot, so I use another measurement), I was getting 880-890 FPS with extreme spreads of 10 or less, with five shot groups at 70 yards a half inch or less repeatedly. This was plenty of power for long-range squirrels, but I found that the first shot of the day would be lower and slower than the subsequent shots. Thinking I had some regulator creep overnight, I thought that adding a quarter turn on the hammer tension screw would give enough added hammer strike to overcome the regulator creep without significantly changing the overall string. That didn't work, and my extreme spreads went up to 20. At that point I decided to do a formal tune according to Bob Sterne's directions.
With the wheel on Max and the hammer tension screw all the way in to where I could barely move the wheel, and the regulator at 110 bar, I got 977-990 ft./s. I began gradually dialing down the regulator, looking for a maximum speed of about 915-920, from which I could back out the hammer tension screw and get to about 895, using the valve adjuster to fine-tune down to the upper 880s range. At the outset I was getting wide velocity swings, 30-40 ft./s. As I reduced the regulator it got worse, and by 85 bar I was having variations of 50 or more. The lowest velocities were indeed lower than they were at the higher regulator pressure, but the high velocities were still as high as before. With such wide swings I couldn't tell whether I was on the plateau. I remembered reading somewhere that too much hammer strike for the regulator setting would give wide swings, so I began gradually increasing the regulator again but the variability did not improve. So I tried using wheel on 5 as my "Max" and gradually backing out the screw, and then doing the same at wheel 4, 3, and 2, but I never got rid of the wide swings. By this time I had gone through several hundred expensive slugs and decided to return to the original settings described above while waiting for more slugs. Sure enough, back at the original settings I was back to my original 70 yards zero and back to tight groups with extreme spreads of 10-12.
So I began thinking that these Power Plenum guns have too much hammer to work well at low regulator pressures with these 23 grain slugs. If so, then to use the low regulator pressures I would need a weaker spring or a lighter hammer. Then I reviewed everything I'd read about tuning, and how insufficient hammer for the regulator pressure produces "partial valve lock" and wide velocity swings also. So which was it? Too much hammer or partial valve lock? By going back to my original settings I increased the regulator pressure and reduced the hammer, which makes me think it's an issue of too much hammer for the regulator. I'm now restocked on slugs, so on thinking of trying again starting with the regulator at 125 bar. I may then get a lower velocity to start with.
Yes, I know, I had a good tune to start, and I could simply have dumped the first shot if I hadn't fired in a while, but I want to understand what happened so that I can tune for various projectiles. Any thoughts?
With the wheel on Max and the hammer tension screw all the way in to where I could barely move the wheel, and the regulator at 110 bar, I got 977-990 ft./s. I began gradually dialing down the regulator, looking for a maximum speed of about 915-920, from which I could back out the hammer tension screw and get to about 895, using the valve adjuster to fine-tune down to the upper 880s range. At the outset I was getting wide velocity swings, 30-40 ft./s. As I reduced the regulator it got worse, and by 85 bar I was having variations of 50 or more. The lowest velocities were indeed lower than they were at the higher regulator pressure, but the high velocities were still as high as before. With such wide swings I couldn't tell whether I was on the plateau. I remembered reading somewhere that too much hammer strike for the regulator setting would give wide swings, so I began gradually increasing the regulator again but the variability did not improve. So I tried using wheel on 5 as my "Max" and gradually backing out the screw, and then doing the same at wheel 4, 3, and 2, but I never got rid of the wide swings. By this time I had gone through several hundred expensive slugs and decided to return to the original settings described above while waiting for more slugs. Sure enough, back at the original settings I was back to my original 70 yards zero and back to tight groups with extreme spreads of 10-12.
So I began thinking that these Power Plenum guns have too much hammer to work well at low regulator pressures with these 23 grain slugs. If so, then to use the low regulator pressures I would need a weaker spring or a lighter hammer. Then I reviewed everything I'd read about tuning, and how insufficient hammer for the regulator pressure produces "partial valve lock" and wide velocity swings also. So which was it? Too much hammer or partial valve lock? By going back to my original settings I increased the regulator pressure and reduced the hammer, which makes me think it's an issue of too much hammer for the regulator. I'm now restocked on slugs, so on thinking of trying again starting with the regulator at 125 bar. I may then get a lower velocity to start with.
Yes, I know, I had a good tune to start, and I could simply have dumped the first shot if I hadn't fired in a while, but I want to understand what happened so that I can tune for various projectiles. Any thoughts?
