point of impact shift FIX for FX Impact and Crown (and others')

[bucketboy]

The accuracy is extremely good on both my barrels. And the jam nut allows for indexing. Keeps weight down, and things simple. There may be a better way, but I sure don't know. I can say it works very well, so I cant see any need for change or improvement from where I'm standing.

Change is a good thing, Just look at all the modifications clever guys have done on here to improve or get more power from their impacts mk1’s some of these have now been included in the lates mk2 impacts direct from FX. (Bigger mag, bigger plenum, better reg, better gauges, bigger VP) who knows what improvements will be included in the mk3 impact?

Yes, the jam nut works but that does not mean it can’t be improved on. How would you like a liner locking system that allows for easier barrel indexing, doesn’t put the liner under compression, holds the liner exactly the same each and every time and can hold any length of liner without the extra expense of getting a different length sleeve? I know I would! Which is why I am going to make one



Bb

 

[Jayson]

As I am slowly backing up on this subject, can some one clearly explain what shift seems to be prevalent? Are we talking every five shots, every other day? Once a month? Does it move for a day and then go back? Thanks guys, very curious, I should have asked this before having an opinion ! I shoot a lot! And I just have not really noticed much that was not self inflicted? But I am also tuning and changing often?. Thanks again!

 

[steve123]

Most of the high quality airguns need a total revamp in the way barrels are attached to the receiver, pretty sad really that they are so delicate and the fit is so loose. 

When I look at my USFT's receiver/barrel fit and attachment, and then to X Y Z brands attachment system, it's a bit silly. Very small POI shifts over many years with my USFT, like a click here and there, and I think that has to do with temps or the way the barrel sits in the case??? Well also altitude but that's due to the different station pressure at those other locations.

I have what's called a prefit barrel in one of my firearms, basically it's a easy way of DYI barrel attachment. The barrel is threaded on the outside of it on the chamber end for an inch, so is the inside of the receiver threaded of course. A barrel nut that is threaded on the inside of it screws onto the outside of the barrel, which when tightened against the face of the receiver pulls the barrel away from the threads in the action. this creates a very secure and tight fit! You can put more than 100 ftlbs of torque on it if wanted but 50 is fine but this is for a heavy steel action. 

Yeah for the inexpensive AG's the way the barrel is attached is somewhat acceptable but IMO things need to change in many of the expensive ones. 

 

[tor47]

I do not know if it is a real issue, but there is 2 O-rings in the nut in my .177 barrel. That should in theory keep the liner centered in the nut, but if the O-rings should have fallen out, or disappeared, there is some play in the nut. I do not know if all calibers have O-rings, but if one suspect the liner moving, it is probably not a bad idea filling up the gap some more.

 

[nomojo65]

123, your talking about the savage barrel nut arrangement but for Airguns! tor47 I agree with you my mk-2 has 2-o-rings on the jam nut plus if you use the 3 o-rings between the liner and sleeve that “should” help also? I suppose the sleeve could be loose on the thimble, but that would be very unusual 

 

[frank320]

With regards to impact shift, let me add that after a rebuild/tear-down/barrel/shroud re-install, i have found a need to let the system "settle" into its groove. Do not re-zero immediately after a rebuild and expect the zero to stay there. Shoot about 30-40 pellets, allow barrel whip and the natural barrel harmonics to settle the rifle, barrel and its components into its steady state, and if possible, let the rifle settle overnight in the shooting position. I sometimes stress the barrel system around by hand to help it settle. Then, re-tighten everything once more and then re-zero this time for good. 

This method has proven reliable for my FX Impact(floating barrel, POI unchanged for the past year ) and also my W.A.R Flex 357(tensioned barrel, POI unchanged for the past 6 months since rebuild). The whole rifle is a system and the barrel is a part of it. Need to let the barrel settle into a solid foundation before you zero for good. Long range precision powder burner shooters do the same by bedding their action/barrel to their rifle stock.

If POI still changes frequently, then you have a different issue on hand.

 

[steve-l]

I suffer the same symptoms with my Crown. It is virtually impossible to tell just what is moving around, but something is for sure. I like the suggestion, so I'm going to use it, but I am also going to use a bottle to shroud clamp as well. The barrel to receiver is good, but it is frail and the slightest bump on the barrel could throw off the POI.

 

[Airgun-hobbyist]

I suffer the same symptoms with my Crown. It is virtually impossible to tell just what is moving around, but something is for sure. I like the suggestion, so I'm going to use it, but I am also going to use a bottle to shroud clamp as well. The barrel to receiver is good, but it is frail and the slightest bump on the barrel could throw off the POI.

I'd go with one modification at a time. That way testing results after taping the barrel to remove the nut tolerance slop is not skewed because of the barrel band installation.

Once the taping procedure is done the tension nut doesn't need to be overly torqued anymore; so it doesn't induce flex in the barrel.

 

[AirSupply]

With the Impact remember you also have the orings that support the barrel on the outside of the shroud. 
These are O-ring 14x1,5 NBR 70. I change these to NBR 90 as I feel they hold the barrel a little tighter. 
If you are changing swapping barrels regularly these will wear and of course can give you a change in your poi. 
I have seen earnest puts another grub screw in that block on both sides to keep it tight. 

 

[bigHUN]

With regards to impact shift, let me add that after a rebuild/tear-down/barrel/shroud re-install, i have found a need to let the system "settle" into its groove. Do not re-zero immediately after a rebuild and expect the zero to stay there. Shoot about 30-40 pellets, allow barrel whip and the natural barrel harmonics to settle the rifle, barrel and its components into its steady state, and if possible, let the rifle settle overnight in the shooting position. I sometimes stress the barrel system around by hand to help it settle. Then, re-tighten everything once more and then re-zero this time for good. 

This method has proven reliable for my FX Impact(floating barrel, POI unchanged for the past year ) and also my W.A.R Flex 357(tensioned barrel, POI unchanged for the past 6 months since rebuild). The whole rifle is a system and the barrel is a part of it. Need to let the barrel settle into a solid foundation before you zero for good. Long range precision powder burner shooters do the same by bedding their action/barrel to their rifle stock.

If POI still changes frequently, then you have a different issue on hand.

Totally agree. Take the physics laws in consideration, the thermo expansion is a game changer. No engineering solution can replace using a common sense.

The liner length can contract/expand differently at 15C or 20C or 25C or 30C temperatures, only mathematical formula can predict the amount but no torque value as a solution in real life.

Especially if you grab the shroud and tighten it just for a hair but the compensator will drive the liner nut. You must feel that "give" with fingers and that 1/4 turn or 1/2 turn mentioned in the pdf is for pure information only.

And btw I melt all apart (separated the shroud and compensator) just days ago and no oring in my 25 cal compensator that could "float" the liner length a little bit, these parts sits there metal on metal contact. I put an oring 9x1.5 only to try how that idea may work on its own but ones I screw on the entire shroud I had no more control about longitudinal tension over a liner. So ended up removing that oring.

I like this idea with a teflon tape I go try it now. But then put a teflon tape both on liner end thread also the other end that screws into the compensator thread as well...

 

[milanilo]

if ur issue is not solved yet check the pellet probe 

check if its tighten 

 

[bigHUN]

Anybody can confirm if there is any Oring in between the liner and the stainless lock-nut on a 700mm 25 cal barrel? Because I have none, and the FX Liner Schematics doesn't show it neither.

But, the liner lock nut recessed cylindrical pocket is not sharp on the bottom but it have some 1mm radius (or about that size), that makes me think the liner shall not touch metal-on-metal?

 

[mr_analyser]

Anybody can confirm if there is any Oring in between the liner and the stainless lock-nut on a 700mm 25 cal barrel? Because I have none, and the FX Liner Schematics doesn't show it neither.

But, the liner lock nut recessed cylindrical pocket is not sharp on the bottom but it have some 1mm radius (or about that size), that makes me think the liner shall not touch metal-on-metal?

My Impakt MK2 .25cal PP doesnt have an oring or any groove in the Jam nut.

 

[Removed]

Surprised someone as smart as Ernest hasn't figured out how to tension the barrel instead of compressing it... a reliable and proven method.

MOD EDIT: FACTS AND SUGGESTIONS WITHOUT INSULTS PLEASE

 

[Dude]

Let's face the facts - Most airguns are built like toys. Some use cast zinc parts and others use clamshells to hold internal components together. And let's not forget about the grub screws that hold critical parts with and without dimples....

 

[daxrush]

I tried the teflon on my M3 .25 600mm. so far no more weird PIO shifts.

Complaints about random points in impact shift while retaining tight group size has been a fairly common theme. The remedy has been to tighten the barrel retaining nut. However I have discovered the reason for the shift and a true remedy that doesn't require barrel bowing torque. Follow along, I'm going to be a bit long-winded...

The cause of the group shift is barrel whip. The barrel can shift within the retaining nut .005" or .013mm, measured with two different barrels. When tightened the barrel relies on tension between the flat top of the barrel and a flat ledge inside of the nut. Again this can move around .005" or .013mm, having tension in one position for a while then barrel whip causing the shift within the retaining nut to move and lock up for a while in another position.

Math says this amount of shift is equal to 1.5" at 100 yards or 41.66mm at 100 meters, based on about a 12" distance between the scope cross hairs and the tip of the barrel on a 600mm barrel. The angle Google calculated is .023873240082231.View attachment 67034

Upon assembly of the barrel liner and retaining nut a couple wraps of Teflon tape or a single wrap of clear adhesive tape removes slop within the side walls of the nut and permanently centers the barrel within the nut.

I tried the teflon on my M3 .25 600mm. so far no more weird PIO shifts. Hope it keeps working like this . Thanks for the tip