Tuning Impact MKll PP finally went off its nut

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Vetmx

Vetmx

Member
Dec 4, 2017
5,894
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Pa, United States
I had a 935fps slug tune for almost 6 months. It was a battle keeping the tune with having to swap crappy gauges for new ones that read completely different and barrel cleanings that you have to put everything back together with the same torque. Recently I swapped scopes and noticed the gun was shooting right at 100. Plus the accuracy sucked. So I cleaned the barrel and did some indexing. Got the left right figured out but accuracy still sucked. Plus, and this is the weird part, I picked up 15fps with slugs but not pellets. This was without touching any setting. The fps gap between my pellets and slugs shrunk. I finally went back to the beginning for tuning for slugs and it’s been a 500 slug head scratcher. Tonight I went with a 130b starting point because that’s what I alway hovered around. I hit 3 accuracy nodes as I descended in velocity but they failed miserably when I jumped to 50yrds. Finally when I had the gun down to about one line on the valve I struck gold. Crazy accurate at 30, 50 and 100. But the gun is at 850fps. That’s -125fps from where I started. I always shot the 26gr slug at 935fps at 130b with great accuracy. I can’t figure out what happened. The only reason I went as low as 850fps was I was starting to believe my liner went to crap. I have been on this gun for two days straight because I was taking it on a trip with me. It never took me long if I took my time to find a good slug tune. When I got the gun I found one at 890fps at 100b then at 920fps then my final tune at 935fps. Do you guys think there is anything internally going on with my gun that is causing it to fight me now?
 
Vetmx, this just happened to me. A .22 700mm Superior liner. Very accurate tune for 23 gr slugs at 925 fps, for months, then my accuracy went to pot. Chrono says I GAINED 15 fps. Had my reg at 95, but for a while it looked more like 90, hard to tell with FX gauges of course, now back to 95. Head scratcher. I suppose a small drop in reg pressure could, if your hammer preload is closely balanced to the reg, cause a significant change in lift or dwell of the valve. And you recently changed gauges, so who knows what your previous reg setting really was?
I have noticed that there is lateral slop in the alignment of the pic rail, and have gotten huge lateral changes in POI after reassembly til I figured that out.

Little changes in velocity have shown up for me on target as surprisingly large lateral POI change, not just vertical.

I replace my breech seal after every cleaning. After a good clean I see a velocity drop of about 40 fps for a while.

Right now I'm thinking it's variation in hammer strike. The edge which engages the sear is a little scalloped from forgetting to close the loading lever before firing. It's mild, but I replaced the hammer last night. Could also be that something is slowing hammer movement, like dirt, so I wiped down what I could while replacing the hammer but may need to do a complete tear down and clean.

They say friction is minimal with pellets compared to slugs, so it follows that once the pellet starts moving it's less sensitive to pressure changes than a slug.
 
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What’s funny is my gun being stubborn to tune happened after swapping scopes. Went from a 34mm to a 30 and used the same brand mount, just moved it from the front of the rail to the back. When I changed my reg gauge I was curious how far off it was so I didn’t want to touch my reg to drain the plenum. Fortunately with an Impact if you shoot it down close to the regs set point, when you untwist the bottle the plenum automatically drains. That’s when I found out that the tune I thought I had at 117b was really a 130b tune. I’m not going to touch the gun for a couple days for fear of throwing it off in the weeds. I’m leaving for my trip with a couple more reliable guns but will keep checking this topic for ideas on my approach when I get back home. I just found it unusual that I had to close off the valve so much to find accuracy even at 50yards. Hell, I don’t think the gun needs a silencer at that 850fps setting. 
 
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Maybe old man crazy here but I'm finding barometric pressure and temperature swings have a lot to do and don't even think about elevation change. Now lube age on bullets or density of lube because of temp. OMG I'm going crazy LOL. Oh and don't forget the two xtra beers I had last night. 

Depending on range I want to shoot( for me it's varmints) unless I have a point of impact change the critters still are in happy hunting ground experience.
 
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I should have gave more info on my gun. It’s a stock 600mm MK2 PP in 25cal. The only mods are to the barrel. I always used a CF sleeve and the shroud is tensioned to 40 inch pounds. That’s what I’ve been using since January and it’s been Taipan consistent. On the times I do pull the liner to clean it, I guess to test fate, I have an indexing mark on the liner and everything gets put back together with the exact torque. Besides gauges, the scope was the only change I did to the gun. I purposely didn’t mod the gun because I wanted to see if I could have a gun like my other guns that I don’t have to treat with silk gloves. I knew what bit me with my last Impact and it was being too quick jumping from one thing to the next. I have been very meticulous with this one in the quest for every day all day consistency. I was there for months. But not anymore. I didn’t even brag about it for fear of jinxing myself. 
 
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Vetmx, did you chrono right before you changed scopes, or months ago?
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I haven’t shot across a chrono in a while. But a month or so ago, right before I swapped my reg gauge I drove 300 miles to my camp, hopped out, assembled the gun and shot this group at 94yards. Maybe when I swapped gauges I didn’t hit the exact reg pressure for everything jive at longer distances. Maybe it was the beginning of my downward spiral. I just can’t remember if I did any shooting over 60yrds after the gauge change. I did take plenty of raccoons and mice at 60. I keep good records of when I touch the gun. Just no good records of when I shoot it from the bench because it’s so sporadic. 
 
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 No, it’s a Nightforce and my groups at 30, 50, and 100 with the gun choked down to 850fps are stellar. So no scope, parallax or mount issue. Maybe I’m just off a couple bar on the reg. What has me perplexed is as all you impact owners know, when you have that valve adjuster wound way in, you’re just spitting out a small quick shot of air. But my gun is at the extreme. I’m well over 100fps from where I started winding it in. I’m just wondering if I’m experiencing some kind of blow by in my barrel system that is disruptive to my slugs until I choke the gun almost completely off. I do appreciate the input guys. 
 
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Laying here I think I’ve come up with a plan. When I get home, I’m going to reset my HS and my VLA to the measurements when my gun shot it’s best. Then I’m going to set my reg at 125b and shoot groups. Then adjust the reg to 126b and do the same until I get to 135b. If that doesn’t fix it I might put the original scope back on. It’s still in its one piece mount and will take 30 seconds. If it becomes a tack driver I will try to film the look on my face. A gun that’s that sensitive to harmonics will cause me to debate selling it. My Taipan with two totally different scopes, with and without night vision attached, sends slugs right where I aim all the time. 
 
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