Cricket bullpup POI shift ?

[mubhaur]

My friend recently purchased Cricket .25 bullpup. It's very accurate out to 100 yards but when he fills the gun and shoots the POI and POA is same but after around 30 shots the POI shift. Group remains tight.

I personally chronied his gun and it is very consistent throughout the complete shot string of 78 shots but still shifts POI.

Before doing chrony I was under the impression that regulator has some problem but this has not been the case. 

Then I studied on some forums that almost all bullpups show similar behaviour if their barrel and air tube are attached to each other by means of clamps. And regretfully most of the bullpups has this kind of set up.

This is because when the air pressure reduces the air tube shrinks a bit and when it is increased the air tube expands a bit. This behaviour affects the barrel as well and in result the POI shifts a bit.

If this thesis is correct is there any remedy?

This behaviour becomes noticeable when shooting beyond 50 yards. Up to 50 yards it is quite negligible. 

I also attach the shot string.

 

[weatherby]

Hey Umair, I think you nailed it.
I've seen this phenomenon more often in guns with airtubes and clamped barrel bands.
Expansion of the air tube definitely can change POI at longer ranges.

 

[Guest]

This seems to be a known issue to owners but not talked about in the community. Hog out the clamp so the barrel can't touch it regardless of pressure and you will see less shift. the next trick is to keep the shroud from contacting the breech the block to eliminate the remainder of your shift.

 

[intenseaty22]

I can tell you that mine had a POI change that was confirmed to be due to changes in ambient temperature. Specially in the Spring and Fall. I posted on this before. 
Mine is a Mini Carbine though, and has a free floating barrel. I used O-rings to correct the shift, it worked. Don’t know about the Bullpup, not sure if it can be isolated. 
Good luck. 

 

[mubhaur]

"weatherby"Hey Umair, I think you nailed it.
I've seen this phenomenon more often in guns with airtubes and clamped barrel bands.
Expansion of the air tube definitely can change POI at longer ranges.

Thank you Weatherby,

Your opinion is always of great value to me.
I read your post with keen interest as these carry real expertise and knowledge .

I would like to contact you through e mail if you allow me.

My e mail address is 
[email protected] 


regards 

Umair Bhaur 

 

[Rj]

I never noticed it with mine, but wasn't shooting 100 yrd groups either. Ernest has a video with the air tube removal that shows the torque sequence for the tube/ barrel clamps. They don't have to be caveman tight. Just snugged up decently. And IIRC the a very tiny gap at the barrel and reciver. Search a couple of his videos for cricket teardown.

IMHO anything with a barrel band has more of a chance for poi shift than a free floated barrel of correct diameter. If you think about it, JMO...A pup less than a long gun. The scope is attached to the same clamp that holds the tube and barrel and should move somewhat together. Where a rifle has the scope on a fixed receiver where the tube and barrel can move independently from one another. Again, just an opinion.

Hope you get it figured out.

Rod

 

[Bigalex]

In Russia, this problem has been solved for a long time, here is an example of the description
"The secret is simple: - Eights should be clamped so that the box without effort, squeezing, and literally under its own weight sat on the barrel-resist in the entire range of pressures. The tightening made 190 atm. Then blew to 120 and blew to 250-260, checking periodically the landing of the box. It turned out that the front eight is tightened tightly, but without fanaticism, and on the back screws quite a bit, so as not to fall out."
https://airgun.org.ru/forum/viewtopic.php?f=6&t=49124&start=120

 

[mubhaur]

"Bigalex"In Russia, this problem has been solved for a long time, here is an example of the description
"The secret is simple: - Eights should be clamped so that the box without effort, squeezing, and literally under its own weight sat on the barrel-resist in the entire range of pressures. The tightening made 190 atm. Then blew to 120 and blew to 250-260, checking periodically the landing of the box. It turned out that the front eight is tightened tightly, but without fanaticism, and on the back screws quite a bit, so as not to fall out."
https://airgun.org.ru/forum/viewtopic.php?f=6&t=49124&start=120

I would like to read this in a bit more simple words to understand.
box ?
190 atm?
120?
250 to 260 ?
under it's own weight ?

 

[Bigalex]

190, 120, 250-260 bar.
box is a receiver

 

[mubhaur]

"Bigalex"190, 120, 250-260 bar.
box is a receiver

Thank you.

any one can explain all that a bit more? 
I understand to some extent.

thanks Bigalex

 

[Bigalex]

the tightening of the screws that hold the barrel and the tank occurs at a pressure of 190 bar, while the front 8 with sufficient reinforcement and the rear 8 is easy only for holding the barrel. So rifles are collected in the service.

 

[Bigalex]

 

[mubhaur]

"Bigalex"the tightening of the screws that hold the barrel and the tank occurs at a pressure of 190 bar, while the front 8 with sufficient reinforcement and the rear 8 is easy only for holding the barrel. So rifles are collected in the service.

Thank you sir 

 

[Rj]

Maybe this will be of some help.