Left to right POI shift - PCP tuning

Left to right POI shift - PCP tuning

Joined: October 10th, 2008, 8:50 am

August 8th, 2010, 10:31 am #1

Question for you PCP tuners out there. Have you ever experienced a POI shift that moves from left to right or vice versa as the pressure in the gun changes? Not talking a lot of shift, but enough to prevent one-holing at 25 yds. I have tried floating the barrel to no avail. Wondering if it could be a result of overpowering the valve and having a hammer spring / valve spring / valve port combination that is wasting gas and causing the spin of the pellet to change as it leaves the muzzle. Right now, I have my Disco .22 tuned to give between 775 - 875 fps depending upon power adjuster setting w/ CPH 14.3's.

Getting ready to tear her down again and do a re-crown at least. Any thoughts as to what else while I'm in there are appreciated.


Sam B

"Happiness is a cool gun..."

Disco .25 ~ Disco .22 ~ Disco .22 Pistol ~ 2250
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Joined: November 17th, 2006, 3:51 am

August 8th, 2010, 1:59 pm #2

Have had certain pellets that demonstrated a tendency to shoot left or right of zero. Several pellets that would shot vertical stringing, but the chronograph showed larger velocity variations and the scale showed weight variations, so they pretty much had to string up and down. Were a few pellets that would nearly always string horizontally, but that tendency didn't show up with other pellets.

Can try a "hog tie" test, it's reversible and free.

1.Remove scope.
2.Place two short lengths of wooden dwoel, about 1/3" diameter, on each side oof the barrel just ahead of the reciever (right where that rear sight mounting sleeve is located). I used those little pencil stubs used for score keeping).
3. Wrap all around the rifle in a 2" strip with Saran Wrap (plastic wrap)..this is just to protect the stock from step #4.
4. DUCT TAPE...wrap it TIGHT around the plastic wrap, about 3 laps as tight as you can make it.
5. Remount the scope and test fire.

Idea here is that the pencil stubs/dowel are going to be wedged into the "v" between the barrel and air tube by the tape, adding stability to the rear end of the action (if the arse end moves, groups will shift). It also adds stability to the stock/action fit (bedding). It's not rock solid, but if it helps stablize the shift in impact, then you know one of two directions to take.

If it helps groups, it may have been the stock shifting in it's bed, the fit of the action to the air tube, or the fit of the barrel to the action.
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Joined: December 24th, 2006, 2:06 pm

August 9th, 2010, 6:11 pm #3

Question for you PCP tuners out there. Have you ever experienced a POI shift that moves from left to right or vice versa as the pressure in the gun changes? Not talking a lot of shift, but enough to prevent one-holing at 25 yds. I have tried floating the barrel to no avail. Wondering if it could be a result of overpowering the valve and having a hammer spring / valve spring / valve port combination that is wasting gas and causing the spin of the pellet to change as it leaves the muzzle. Right now, I have my Disco .22 tuned to give between 775 - 875 fps depending upon power adjuster setting w/ CPH 14.3's.

Getting ready to tear her down again and do a re-crown at least. Any thoughts as to what else while I'm in there are appreciated.


Sam B

"Happiness is a cool gun..."

Disco .25 ~ Disco .22 ~ Disco .22 Pistol ~ 2250
brand-new .177 Marauder. It would shoot the first 12-15 pellets off a full fill (2800 psi) just fine, about half-way through the second magazine the POI would start moving to the left, and the group size would open up. Re-zero it after 20 shots or so then refill, guess what? POI jumps back to the right, then starts to march left as the pressure in the tube drops. Never did figure that one out, played with the barrel band, cleaned/inspected the barrel, nothing obviously wrong. It went back to the vendor.
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Joined: October 10th, 2008, 8:50 am

August 10th, 2010, 10:22 am #4

Question for you PCP tuners out there. Have you ever experienced a POI shift that moves from left to right or vice versa as the pressure in the gun changes? Not talking a lot of shift, but enough to prevent one-holing at 25 yds. I have tried floating the barrel to no avail. Wondering if it could be a result of overpowering the valve and having a hammer spring / valve spring / valve port combination that is wasting gas and causing the spin of the pellet to change as it leaves the muzzle. Right now, I have my Disco .22 tuned to give between 775 - 875 fps depending upon power adjuster setting w/ CPH 14.3's.

Getting ready to tear her down again and do a re-crown at least. Any thoughts as to what else while I'm in there are appreciated.


Sam B

"Happiness is a cool gun..."

Disco .25 ~ Disco .22 ~ Disco .22 Pistol ~ 2250
Too late to send back, this one came from the parts bin. I suspect that I either did a lousy job crowning, or have messed with valve / hammer spring combo in a bad way.

Stock is a James Linthicum, and I think that the fit is good, but will have to try the suggestion to tighten the action to the stock.

was your m-rod one with the fixed barrel bands or one of the newer floating ones?



Sam B

"Happiness is a cool gun..."

Disco .25 ~ Disco .22 ~ Disco .22 Pistol ~ 2250
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Joined: May 25th, 2008, 4:38 am

August 12th, 2010, 4:42 am #5

Disco gradually drops POI after 20 shots...but no left/right POI shifts..it is tuned to shoot 906 FPS using JSB 14.3 gr....factory stock,with barrel band in original position...and factory muzzle brake..

If you still have your Discovery stock,I'd try that first and compare results...
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Joined: January 30th, 2005, 11:00 pm

August 12th, 2010, 11:29 am #6

Question for you PCP tuners out there. Have you ever experienced a POI shift that moves from left to right or vice versa as the pressure in the gun changes? Not talking a lot of shift, but enough to prevent one-holing at 25 yds. I have tried floating the barrel to no avail. Wondering if it could be a result of overpowering the valve and having a hammer spring / valve spring / valve port combination that is wasting gas and causing the spin of the pellet to change as it leaves the muzzle. Right now, I have my Disco .22 tuned to give between 775 - 875 fps depending upon power adjuster setting w/ CPH 14.3's.

Getting ready to tear her down again and do a re-crown at least. Any thoughts as to what else while I'm in there are appreciated.


Sam B

"Happiness is a cool gun..."

Disco .25 ~ Disco .22 ~ Disco .22 Pistol ~ 2250
Sam I have had that same problem, and it is mostly caused by a poor barrel. I bet if you slug that barrel you will find it loose at the muzzle, or looser then the breach end. A choked barrel will stop that problem. Or if you could regulate the pressure to the valve, to keep fps constant, it will shoot ok.
Bill...
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Joined: October 10th, 2008, 8:50 am

August 13th, 2010, 10:14 am #7

Sounds like some of you have tuned higher than I am, and only getting the expected vertical stringing. Will try the barrel to start. This build of the .22 has a different barrel as the blueing was in better shape. Each barrel has the pellet it likes best, and given that the left to right seems to disappear when I use tight Eun Jins, that would seem to point to the barrel as well. Happy to stay with the fat heavies in the .25, but not so much in my .22. More as it comes. Thanks!

Sam B

"Happiness is a cool gun..."

Disco .25 ~ Disco .22 ~ Disco .22 Pistol ~ 2250
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