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

[Airgun-hobbyist]

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.

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.

 

[TNT19]

Impressive deduction. That had to of taken some serious time to figure it out. 

MOD EDIT: Uncorroborated conjecture removed

 

[IguanaPolice]

Maybe FX should put a notch onto the liner so that it indexes to the barrel and won't rotate. Or a series of notches so that it can be indexed purposefully to find the sweet spot and still not rotate

 

[tor47]

That is interesting. But it may be a litle problematic if the liner is indexed, shooting with high POI at 12 o`clock. Would not that make the whole liner rotate as you tighten the nut?

 

[Vana2]

Thanks for posting!

Makes sense, notes made and squirreled away for reference.

Without having seen one, I wonder if a chamfer (in the nut and on the liner) to center the two parts would be a permanent solution.

 

[Airgun-hobbyist]

Maybe FX should put a notch onto the liner so that it indexes to the barrel and won't rotate. Or a series of notches so that it can be indexed purposefully to find the sweet spot and still not rotate

If they taper both the end of the barrel and the inside of the nut they will center themselves..

 

[TNT19]

I watched a recent video of rossi touring the fx factory, and they stated they test every guns accuracy before it ships. Makes one wonder how they would miss something that important on such a high end gun . Seems more and more people are finding things off right out of the box. I have a kral, gauntlet, and the rti priest 2 till recently, and have zero issues on any one of them right out of the box. I have modified all but the priest 2 for more power etc. The kral puncher breaker bullpup is 3 years old and 3k pellets through it and the only thing I changed was the cocking lever. The gauntlet only thing was the air stripper was cracked. The priest absolutely zero issues was moa at 100 yard right out of the box. 

MOD EDIT: Uncorroborated conjecture removed

 

[Airgun-hobbyist]

That is interesting. But it may be a litle problematic if the liner is indexed, shooting with high POI at 12 o`clock. Would not that make the whole liner rotate as you tighten the nut?

The Teflon thread sealant tape allowed the barrel liner to remain in it's position as the nut was tightened because it wasn't too thick. The adhesive tape was thicker and a tight fit, so the liner spun. I index also, but I think this may change that because of it centering the liner nearly exactly the same every time.

 

[TNT19]

@Airgun that's awesome you got it right away and didnt have to struggle with it. There are lots of others I am sure loving this post. I love this forum and the community as everyone shares what they have discovered. 

 

[Airgun-hobbyist]

Thanks for posting!

Makes sense, notes made and squirreled away for reference.

Without having seen one, I wonder if a chamfer (in the nut and on the liner) to center the two parts would be a permanent solution.

I'm not positive about other manufacturers' liner retention systems, but this type of centering solution may benefit them as well.

 

[Airgun-hobbyist]

I watched a recent video of rossi touring the fx factory, and they stated they test every guns accuracy before it ships. Makes one wonder how they would miss something that important on such a high end gun . Seems more and more people are finding things off right out of the box. I have a kral, gauntlet, and the rti priest 2 till recently, and have zero issues on any one of them right out of the box. I have modified all but the priest 2 for more power etc. The kral puncher breaker bullpup is 3 years old and 3k pellets through it and the only thing I changed was the cocking lever. The gauntlet only thing was the air stripper was cracked. The priest absolutely zero issues was moa at 100 yard right out of the box.

Do not place blame with FX, it not something that happens "from the factory" with a new rifle necessarily. It is something that happens randomly and not that often really. Uncommon factors include lack of adequate tension, wear from use, heat expansion (like scope shift) causing loosening of parts, and high power barrel whip.

 

[TarheelScott]

Do not place blame with FX, it not something that happens "from the factory" with a new rifle necessarily. It is something that happens randomly and not that often really. Uncommon factors include lack of adequate tension, wear from use, heat expansion (like scope shift) causing loosening of parts, and high power barrel whip.

Plus 1 for the sharing of info. and this very true statement above. Thank you for sharing with us .

 

[Long_Gun_Dallas]

Having superllight STX barrels, there is no retaining nut. So, this issue does not exist.



Good fix, though

 

[Jayson]

++2 agreed,And very much appreciate the concept, but a myth in my 7 Fx guns and 12 liners, first, my money on poi shift would be parallax, inconsistency with cheek weld/scope set up and mediocre glass, then Second, a solid user liner reassemble technique?Fx should put a torque recommendation on that nut! The Continuum doesn’t even have the nut, just sits there and no shift noted? Just my bs opinion Thanks

 

[tor47]

According to the liner manual https://fxairguns.com/wp-content/uploads/2019/06/liner-replacement-eng.pdf, the nut is not supposed to be tightened more than aprox a quarter turn.

 

[Airgun-hobbyist]

++2 agreed,And very much appreciate the concept, but a myth in my 7 Fx guns and 12 liners, first, my money on poi shift would be parallax, inconsistency with cheek weld/scope set up and mediocre glass, then Second, a solid user liner reassemble technique?Fx should put a torque recommendation on that nut! The Continuum doesn’t even have the nut, just sits there and no shift noted? Just my bs opinion

Thanks

I have no doubts that some point of impact shifts happen due to parralax and hold/weld techniques. I have experienced it. These issues usually result in larger than normal group size. But they don't explain two tight groups moving just an inch and a half apart from each other. The math shows that if the barrel liner is moving around within the nut it moves the group a pretty specific amount at 100 yards.

 

[heavy-impact]

You want to avoid compression on any barrel since it magnifies vibration. Tightening the barrel nut adds compression not tension. If you could add pulling force (tension) it would make the barrel more rigid, but there is no means of doing that. The best we can do on an impact is a tiny bit of compression so I agree that the teflon tape is a good idea and only tighten the barrel nut enough to remove any play.

 

[Long_Gun_Dallas]

The Continuum doesn’t even have the nut, just sits there and no shift noted? Just my bs opinion

Thanks

It's got a jam nut at the base. The end does just float. Liner has some shroud spacers it rides on

 

[bucketboy]

That’s well thought out.

Personally i think the jam nut is a cheap fix to keep the cost down (or profits up) I can think of a couple of better engineering solution that would hold the liner without putting in any unnecessary tension and would hold it 100% true regardless of the liners rotational orientation.



I am not a FX hater,I have an impact, while I am generally impressed by the quality of the gun there are several areas that can (and should) be improved on, the jam nut being one of those.



Bb

 

[Long_Gun_Dallas]

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.