New Marauder POI Shift Left to Right Across Shot String

[Deanwa]

I have a new Marauder Synthetic .25 cal that now has about 300 shots through it. I am noticing that the Point of Impact consistantly shifts from left to right about 1.5" at 40 yards across even just a 16 shot string.

So at the beginning of the shot string it is 3/4" left of bullseye and at the end of the shot string it is about 3/4" right of bullseye even though the velocity is the same. Up and down is unchanged and the gun is otherwise very accurate. This is the factory tune which I thought was perfect according to the chrono and small group accuracy. 

Is there a way that I can minimize this issue by fine tuning? Other suggestions?



Here is a sample of my shot strings, velocity measured at 10'. 25.39gr JSB Kings.

Pressure 3050

844 POI is 3/4" Left of Bullseye @ 40 Yds

843

857

851

869

869

862

861 POI is dead on Bullseye

859

857

857

860

853

852

842 POI is 3/4" Right of Bullseye @ 40 Yards

838

Pressure 2200



Thank you in advance for your help!

 

[starlingassn]

make sure the barrel band isnt pressing against the barrel.

if it is,that could happen as the pressure drops.

had that happen before in my marauder

 

[Brazos]

Try some simple things. First is there any wind? If so wait for a day with zero wind. Second are you using a scope level? If not get one. Third ar you using the magazine or a single shot tray. If using a magazine then try it single shot for a while. Forth thing to try is just remove the barrel band. It's just a couple of set screws.

 

[Deanwa]

Thanks for the suggestions so far guys. I ruled out these things so far, barrel band does not contact barrel, no wind, single shot or magazine same, gun level.

I think it has something to do with the time the projectile is spending in the barrel. At the top of the shot string it is a shorter burst of high pressure for a quick jump, and at the bottom of the shot string it is a longer burst of lower pressure. So at the end of the shot string even though the velocity is the same as the beginning, the projectile is in the barrel a little longer after the trigger is pulled.

I am shooting simulated hunting conditions. Sitting back against tree, elbows between knees, this makes for a steady hold for me in hunting conditions but may be vulnerable to the slight change in dwell time in the barrel. Is there any way to minimize this with tuning?

 

[sonny]

how about rifle cant or scope parallax

 

[BigTinBoat]

Thanks for the suggestions so far guys. I ruled out these things so far, barrel band does not contact barrel, no wind, single shot or magazine same, gun level.

I think it has something to do with the time the projectile is spending in the barrel. At the top of the shot string it is a shorter burst of high pressure for a quick jump, and at the bottom of the shot string it is a longer burst of lower pressure. So at the end of the shot string even though the velocity is the same as the beginning, the projectile is in the barrel a little longer after the trigger is pulled.

I am shooting simulated hunting conditions. Sitting back against tree, elbows between knees, this makes for a steady hold for me in hunting conditions but may be vulnerable to the slight change in dwell time in the barrel. Is there any way to minimize this with tuning?

Doubt this is it, let someone else shoot the gun and see if it changes. Don't tell that why you want them to shoot it, or what you are experiencing.

 

[Brazos]

You have an interesting theory. Another quick and easy thing to try would be removing the shroud and shoot bear barrel. If it works then you could at least isolate the issue to the shroud. If your theory is correct then a regulator should solve it.

 

[Vetmx]

I was going to suggest most of the things the guys said when I first read this post. What has me stumped is you said “consistently “. It is very hard to cant or uncant your gun consistently across many shot strings. If it is indeed something your gun is doing consistently, you should be able to put up 4 targets, fire a string of shots at each one and they should all look the same. If that’s the case, I am still stumped. Shrouds or clipping can cause problems but they aren’t consistent like you described. A CO2 gun gets colder as you shoot and that could cause a problem like yours but that’s not the case. A burr on the crown of your barrel can cause fliers, but not a steady consistent left to right swing in your group. 

 

[jwrabbit123]

WOBBLING???



Is your pellets wobbling? here is what I do when I have accuracy issue it may help , First clean gun and tighten everything

Second , if it is multi shot , remove o rings on magazine and drop pellets threw and if they do not just fall threw with their own weight open holes with drill or file reinstall orings ,

third , wash your pellets and dry them and sort by size and weight if you want to rule out ammo , and at this point you should be fine . if not and pellets still wobbling push a few threw with cleaning rod and feel how they go threw bore look closely at rifling in pellets it should contact skirt and heads if it doesn't try different ammo , and it you see gouges you may have a barrel which may need lapping . I had Hatsans with both magazines machined a bit off and I have had many poorly machined barrels which needed alot of attention . I had fx barrels with bad crowns , I had a ataman with a sloppy bore so it could be anything . also if you need to look at barrel use a bore light and good magnifier .I have had shrouds clip pellets as well shoot it with shroud off , take all them cups out and shoot a group , I think this covers it

ALSO if you have a scope cam set up take a few shots filming at high frame rates you may see the dredded wobble , and the wobble may be from a tight magazine as when pellet goes threw it , it bends skirt from it being too tight .( I know you said you used single shot tray but check mags and tune them as well )

this is what I have found in 40 plus years with this problem let me know thanks . 

 

[Deanwa]

What has me stumped is you said “consistently “. It is very hard to cant or uncant your gun consistently across many shot strings. If it is indeed something your gun is doing consistently, you should be able to put up 4 targets, fire a string of shots at each one and they should all look the same. If that’s the case, I am still stumped.

Yes, you are right I think, consistency is the key. It is always about 3/4 inch left at the beginning of the string and about 3/4 right at the end of the string even though the velocity is the same at the beginning and end of the string. I can shoot group after group and that is consistent. Exterior factors like wind would not have this consistency. That is why I was thinking tune and dwell time in the barrel. In forearms terms, this would be akin to changing the burn rate of the powder but adjusting the charge to keep the same muzzle velocity. That quite often results in POI change even with the same muzzle velocity. The slight change in he time the bullet exits the barrel catches it in a different part of the standing wave from vibrations.

I was just hoping there was a way to tune that out at least partially, to be more consistent.

I can shoot 4 shot groups all smaller than a dime at 40 yards. It is just that each group moves from left to right as it gets further down the shot string. (16 shots). And then starts over when I recharge.

 

[alex2865]

What scope r u using.is it a leapers 4x16x44.alex.because they r junk

 

[ackuric]

To obtain the same terminal velocity, your pellet 'dwell' time inside the barrel from its original seating location to muzzle exit is nearly the same. So I wouldn't look to deep into that. If you truly feel this to be the case a regulator would be the only way I can think of to correct it. This keeps the output pressure nearly the same through the shot string.



I run my marauder with an o-ring added snugly between shroud and barrel band. Many people prefer this opposed to the free floated factory style. 



HTH,

-Matt

 

[Makoda]

Mine did the exact same thing, I recently switched barrel to a 177 from 22 and have not checked if it was still doing it but I had the exact problem with mine and it was very consistent 

 

[ranedouglas]

if it were me , i would contact crosman and exchange it. i was able to get a handful of those .25 green mountain barrels from them and the patterns never walked like that - especially as close as 40 yards. i wouldnt spend any extra on that one until you speak with crosman. now , my older original discovery will make a 'U' shaped pattern at 40-50 yrds - but probably not even one inch across....and it's a cheaper barrel.... i was going to mention that i've been much more interested in the .25 mrods now that shot count can be made higher more easily : " ssg tune " ..... yea, man , yours is pretty consistent but that walking almost 2" horizontally poop..... youve only put 300 shots - just exchange it... crosman is ultra cool customer service - the best in the industry. ......and one last thing , i noticed your chrono readings were from ten feet away.... ive shot more than two chronos at less than 2 feet away... i couldnt do that very well .... i just try to use 6" up and 6" away from first sensor now - some co2 guns have to be back farther because of the cloud of gas.

 

[ChuckHunter]

I had a gen 1 that had a similar problem with Kings. And the O ring trick on the barrel band helped some but still not enough...

I installed an HDD on the hammer spring and started playing with different ammo and found out it liked a little longer pellet. 

Barracuda hunters shot very well but when I tried poly mags it was like night and day accuracy wise! It just loved those and took several woodchucks at 60+ yards right in the ear hole....just couldn't get that consistency from the 25.39 kings. 

Never tried the heavies in it but that might be another alternative. 

James from Michigan, 

 

[Vetmx]

I would do what Chuckhunter said, try a different pellet. It’s cheap and it’s nice to have another pellet than a JSB. Seeing how they are not always available when you need them.

 

[Butch]

I had left to right movement of poi on my disc with an elite mounted on top. Every time I would fill the gun it would do as yours is. I tried a lot of different things such as you with great frustration. In the end it was the scope crosshairs causing the issue. I would lay the gun on its side to fill which shifted the crosshairs and during firing it would shift back. 

The way I found it was by tapping on the scope then shooting the target and of course bushnell fixed the scope for me. 

Me personally would of never thought it would have been the scope on a pcp, but scopes break and loosen up with some bumps. 

Hope this helps. 

 

[Deanwa]

Possibly JWRabbit123 is on to something with the wobble. I did notice that the bolt goes into the barrel slightly high, which may be seating the pellet out of center. When I first got the gun, it was blowing the seal out the bottom below the bolt until I put a new seal in. It seals fine now. 

Please see the attached picture and let me know if you think it is off center enough to cause an issue with wobble.

 

[Vetmx]

Wobbling pellets can definitely cause accuracy problems but once again how are you getting pellets to wobble consistently at the same rate through many shot strings. That would mean at the start of every string your pellet is either wobbling or not wobbling. Then as you shoot it progressively gets worse or better. Not likely. Looking at your picture, if the front of your probe has nice rounded edges, it being a couple thousandths high is not causing your problem. An issue like you are having is more easy to solve with something that gets hotter or colder as you shoot. A rifle that does what yours is doing is heat related. Every shot the barrel gets hotter, expands more, and if it is making contact with an imperfection on the left side of the barrel channel it will keep pushing your shots further and further to the right. With a PCP airgun, the only thing that changes is air pressure. You start at 3050 and shoot down to 2200 consistently. As your tube goes through that pressure drop, your groups are swinging, consistently. I would pull the gun completely apart and reassemble it making sure everything is tight including whatever Crosman uses to hold the valve in place. Pay particular attention to how the breech block is screwed to the air tube. If you are reluctant to tear your gun down, you should probably throw on another scope, takes 15 minutes. Then try another pellet. Do the easy stuff first.