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Discovery/Katana POI shifts... Need some advice...

March 29 2013 at 12:15 PM
JohnL57  (Login JohnL57)
YF

Hi, All,
I've been trying to dial in my Benji Katana and could use some direction. I'm getting 1/2" and sometimes 3/8" groups at 50 yards,BUT the POI will shift from fill to fill. Yesterday evening I got it dialed for 50 yds, next fill the POI shifted 3/4" down and 3/4" left. Still a very tight group, 1/2" five shots. I am aware of the tiny breech screw issues and have tightened it down. I haven't put a second barrel band in front of the breech yet, but I guess that's my next step. Have tried two different scopes with no difference. I am using cheap Barska rings and may try upgrading to something better like a BKL. I've done some searching on this and i think I've covered just about everything, but any advice from here would be welcome, That tack driver is so close I can taste it! John

 
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robert
(Login gubb33ps)
YF

Experiment first...

March 29 2013, 9:11 PM 

If you think a barrel band ahead of the breech might help, then use a plastic tie (one of those ties that zip closed and won't back up) TIGHT at that spot and test. Will look ghetto, but will give you an idea if its worth inletting the stock and fitting the band before you start cutting.

 
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JohnL57
(Login JohnL57)
YF

Cool idea! Will give it a try...Thanks!nt

March 30 2013, 12:59 AM 


 
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Art Deuel
(Login ADeuel)
YF

John, have you resolved the issue yet?....nt

April 1 2013, 7:41 AM 


 
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JohnL57
(Login JohnL57)
YF

Haven't had time-I'm hoping to work on it today..got any ideas?nt

April 1 2013, 10:19 AM 


 
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Art Deuel
(Login ADeuel)
YF

Try Robert's idea first, that would be quick....

April 1 2013, 12:02 PM 

If that doesn't fix it I have something you can try. I had one that acted very similar to what you describe.

While you have the stock off to install the tie, check to see if there are any hi spots in the stock that are pushing against the air tube. Ideally you want the air tube to be a free float from the gauge port and forward.

I had one that drove me crazy until I figured it out. Metal has a "grain" just like wood and sometimes it will change with pressure. The one I had would actually move into a very slight curve as the air pressure was increased and then relax as the pressure dropped. This wasn't anything that you could notice simply by looking at it. As the tube moved it also moved the barrel and hence the group would also move.

I replaced the air tube and the gun shoots normal now.
I like to use two barrel bands on the Discos.

ART

 
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JohnL57
(Login JohnL57)
YF

I was thinking of trying a second barrel band with the stock removed....

April 1 2013, 12:10 PM 

Is your setup one band very close to breech and one out near the muzzle? I've got a band with six screws to use at the muzzle-I adjust so the barrel screws just touch. Haven't even considered the stock might be a factor, I'll inspect for contact marks when I tear down. And to think I thought Chinese springers were frustrating! At least this one shows some amazing poitential, though.
John

 
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Art Deuel
(Login ADeuel)
YF

With the barrel sleeve removed...

April 1 2013, 4:36 PM 

I position the rear band about 1/2" ahead of the gauge and the front band about 1" behind the fill port dust cover, seems to work good that way. The big thing is getting the breech mounted solid and the band to add additional strength to the whole set up.

ART

 
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Mach-1
(Login Mach-1)
AR&P

If POI shift is predictable over the air pressure

April 4 2013, 3:13 PM 

Art's idea may be the ticket. If not then look to getting the breech mounted good and flat.

I had one that the POI shifted after each fill or from day to day. That was until I figured out how to get that breech correctly mounted flat to the tube. The POI no longer moves from day to day or fill to fill or even as the pressure changes in the tube.

And your correct... these Discovery rifles are very accurate if everything is put together just right.

 
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Mach-1
(Login Mach-1)
AR&P

Advice on free floating the air tube

April 4 2013, 3:19 PM 

Leave a few inches of wood in place for securing the wood to the gun. You need something for there to tighten up the stock or it will rock on the single mounting screw and that will make things worse.

A piece of 3/4" PVC pipe wrapped with sandpaper works well to remove the wood from the stock. Just be sure to not remove too much.

 
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Daniel
(Login rdsail)
YF12

Opring on the barrel

April 28 2013, 8:45 PM 

I tried this and it fixed all my problems. I use two bands and I added an oring on the barrel on each side to the bands The first band is in the regular spot and the set screws are tight. The second band in about 1.5 from the muzzle. the airtube set screw is tight the set screw for the the barrel very loose. I could actually take it off.

Also, IF you have a TKO. make sure it is secured and that it stays aligned. If you move the TKO while filling and the alignment changes your POI will change.

[linked image]


O-rings I used
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JohnL57
(Login JohnL57)
YF

I'm using something close to that now...

May 1 2013, 12:02 AM 

I worked on getting the front of the breech nice and tight- actually sanded down the transfer port a bit as well as the rubber sleeve that goes between the port and the air tube. I had a friend make a six screw mod to the band near the breech. Interestingly, I found that the band pulled the front of the breech up! I then took a dowel wrapped in sandpaper and worked on the band till it allowed the breech to fit tightly. For the front band I wrapped a strip of soda can tightly around a rod smaller than the barrel then trimmed it so it went all the way around the barrel. Set screw barely touches the soda can sleeve allowing the barrel to float a bit. I'm cautiously optimistic that I've got it shooting consistently, will do a post when I know for sure.

 
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