What is supposed to happen to accuracy if the shroud is removed?

[riazp]

Hi All, 

I have an Impact in .22 and I decided to try it out with the entire shroud removed. This gun has never performed as well as my Bobcat.

The results were quite disturbing as the accuracy went out the window! I have been trying to get this gun to shoot as well as my mk1 Bobcat, 20mm at 50 meters, and haven't been able to do so even at 25 meters.

I have been doubtful of the extending shroud, it has loads of play in the extended and retracted form, while I have been able to address this with well placed scotch tape I decided to see what effect a bare barrel would have.

I shot my worst group ever, I print out 144 18mm circles with 2mm bulls on an A4 sheet and shoot from 250 to about 140 bar, 126 shots, gun is shooting Jsb 18 grains at 863 fps reg is at 125 bar.

I expected the gun to at least touch the 2mm ring every time at 25m, like the Bobcat, but had many shots completely miss the 18 mm ring. My shooting skills are OK, I shoot at least 60 rounds daily since April last year and 120 rounds since the arrival of the impact six months ago

Anyone experienced issues like this before?

I have no idea what to think as it seems as though I may have a very good looking but inaccurate premium airgun.

I have tried:
washed pellets, washed and lubed, lubed, straight from tin, more speed, less speed, changed All exposed orings, set screw is always in place, clean barrel, fouled, have adjustable scope rings, have optically centered the scope, chronied a full string of 126 shots with an ES of about 10 fps, inspected crown, inspected grooves and lands inside and outside under the crown protector, pushed pellets through barrel and inspected, have put over 10000 rounds through the gun, have gone through the troubleshooting guide.

I am going to try a different scope and see if the current scope is not holding a zero, the dispersion is mostly horizontal. Current scope is a hawke 8-32x56.

I really have read just about everything impact related, and am open to any more reading suggestions or ideas. Trying to avoid returning this gun as I love the whole concept, I have even considered taking the Bobcat barrel and fitting it in the impact, but the Bobcat was bought second hand and is out of warranty and shoots too well to mess with.

 

[Auronotcs]

You said everything but single feed pellets without the magazine. Could we possibly see a picture of the groups?

 

[broekzwans]

Have you tried some different pellets and head sizes? (I couldn't find it in your story ) Not every gun likes the exact same pellets.

And have you tried different holds? What I've heard about the wildcat is that it's a tack driver as long as its placed on a bag or bipod and shouldered very losely, maybe the impact prefers the same. If you use the wrong hold for the gun you can tune/modify whatever you want but it'll never perform the way you want to.

For the rest you've basically tried everything else If the gun still isn't performing after the steps above the last thing is to return it I think. 25 meter should be practically ragged groups every time (or one hole if you do your part exceptionally well).

 

[Guest]

A shroud may cause POI shift when installed/removed but if properly aligned it shouldn't cause accuracy to degrade either way.

Does the shroud or shroud cap tension the barrel in any way when it's installed?

 

[riazp]

Hi All,

Pictures bellow

suspect group
how I fixed play between inner and outer shroud 
indoor range shooting setup
group with shroud 
the only size .22 we get here in the land of Matt Dubber
50 meter outdoor range 6 shot group from .22 Bobcat
Bobcat used to shoot that group, not using the moderator

https://imgur.com/g4E5w

 

[addertooth]

The most rigid part of the pipe is it's outer most diameter (in terms of avoiding being bent or vibrating). The smaller the pipe is, the greater the probability it will ring or vibrate when struck. (firing a pellet is like striking your barrel) Your shroud acts as a larger pipe which will normally dampens barrel vibrations. Some guns do improve from having the shroud removed, you just happen to have one which gains a benefit (standing wave vibration dampening) by leaving the shroud in place. Some people "tune" the standing wave characteristics by simply changing how hard they screw down the shroud. There are forumites who have gained accuracy by as little as a quarter turn difference. 

 

[dreuf]

Last idea : barrel polishing with jb bore products. It increased accuracy on my.25 ST and removed some flyers.

How often do you clean your barrel?

 

[Gmg]

I'm thinking that telescopic shroud isn't the best idea for FX impact. Ted and Matt are using fixed shrouds. They also claim their guns have laser accuracy. Coincidence ?

 

[GQ]

My Impact is set with a fixed shroud and barrel. The barrel is longer, so the shroud is.
I dont trust telescopic shroud.
Your accuracy problem may not come from your shroud or barrel.
It can be the probe alignment or loose from inside.
It can be the probe set screw.
It can be the barrel set screw.
Difficult to find for us not having the gun in front of us.

 

[scrane]

The first thing I would do with an inaccurate gun is give the bore a good scrubbing with a rod and a tight brass brush.

 

[zebra]

Nothing is meant to happen to accuracy when the shroud is removed. A heavy LDC might cause a poi shift but it shouldn't effect accuracy if it is properly centered. If it isn't, there would be clipping and it would be visible normally. 

 

[Auronotcs]

"zebra"Nothing is meant to happen to accuracy when the shroud is removed. A heavy LDC might cause a poi shift but it shouldn't effect accuracy if it is properly centered. If it isn't, there would be clipping and it would be visible normally.

Can't some poorly designed shrouds also cause... I'm not sure on the terminology, but it's similar to having a bad crown on your barrel, and can cause bullet instability due to uneven expulsion of gasses at the end of the barrel? But that doesn't explain why with the shroud removed the gun is sub-optimal. I

 

[chasdicapua]

Both guns (Hatsan Galatian .25 and RAW TM 1000) I've had with shrouds got slightly better (tighter groups) with the shroud off. 

 

[scottblair]

The one thing that I haven't read is trying different pellets like mentioned above. I own many high end AG's, not all of them like the same pellets. Maybe you should order some different types, and head sizes from the web sites. just my 2¢

 

[sniperlabyo]

Try a larger oring on the 2 barrel bushing, there are 2 oring per bushing.

this will minimize barrel movements every time you cock the gun, the cocking bushing ride on the barrel and add force right and left.

over time stock oring loosen up. If it's to easy the side barrel In place you need to replace them,

i use viton oring thats is harder and next size up. So barrel is hard to slide in and vary hard to pull out---sometime I have to use a delrine rod at the rear to get it started moving then pull barrel out.

also those 4 cape screw for the barrel bushing gets loose-- it will affect your accuracy, the rail and scope is mounted on those bushing. You have to use med thread seals on them. 

 

[Scirroco]

Hi ernest
May i please ask what is the size of the o rings that you use ??
Rgds steve

 

[Verve]

1. Remove the rail and check main plate screws for loose ones - if any are loose, then loosen them all, insert barrel and then tighten them all.
2. Check the Smooth Twist part of the barrel for machining issues
3. Get one size up o-rings in the bushings that barrel slides through

I know you've been trying to fix accuracy issues as long as I have and if nothing is working, put it back in the box, as I have, until you decide what to do with it.

 

[riazp]

Hi All, 

Thank you all for the suggestions!

I will try to address all the replies:

Pellet choices is South Africa are limited to JSB and H&N, and always in 5.52 only, I wish that I could find different skirt sizes...

I am reluctant to put any rods down my barrel, but at this stage I will try to find a carbon or plastic coated rod and a .177 brass brush, some JB and a bore mop to hand polish, maybe being extra careful around the twist/choke part, have considered using brass or metal polish/rouge, I suppose JB paste is tried and trusted so will just go find some!

I have replaced all exterior o rings with Viton, except the inner barrel 5.5*1.5, my stock of 2*1 and 5.5*1.5 in viton were not close enough to specifications to provide a good seal, so have reverted to nbr 70 in those applications.

I have not tried the next size up or down to create a better fit, I am game however, I suppose the next size up for outer barrel and the next size down for the barrel guide blocks can't hurt.

I think that the screws under scope rail are snug, however I think that it's worth a drop of blue thread seal so I will see to that.

The shroud, oh a kingdom for a .22 shroud that is without suspicion! Poor fit in stock configuration, fixed with two layers of scotch tape, I suspect clipping on the air stripper as well as on the baffle pipe. I removed the baffle pipe, inserted acoustic Matt and a single Derin baffle bored out to 7.65 mm. This made it quiet but did nothing for accuracy, or should I say that it improved accuracy marginally.

Right now best best accuracy is from a bare stock shroud, stock baffle removed and stock end cap. It barks like a scalded dog.

@Verve,
I feel your pain, and it's good to know that I don't walk alone, how did you know that the next stop for my gun was packed up and ready for return! I knew that when I bought the gun that money was no object, I was lucky, but I am running out of things to buy to make it better, and I am seriously thinking of writing a letter to FX asking how much more I need to spend to make the gun perfect, as I am prepared to spend it, if they can fix it! Have even considered rebarreling but still need a tuned shroud it seems.

@Ernest,
You are such a star! Thanx for the help with the Wikas, and for the suggestions, if I still lived in North America the gun would have been on your workbench a long time ago.

@ Everyone else,

Thanx for providing perspective, I have been pulling hair out and your suggestions have vetted a lot of what has been tried already.

If anyone has any contacts or friends at FX please let them know that there is one Impact owner who is still prepared to persevere and spend more if need be to get an accurate .22 lmpact with no issues, it has to be possible...