<< Previous Topic | Next Topic >>Return to Index  

Benji Discovery/Katana POI shifts.. still not there..now what?

May 5 2013 at 10:14 PM
JohnL57  (Login JohnL57)
YF

Ok, so I cleaned up the transfer port and made sure the breech was tight, couldn't slide a thin strip of paper under the breech, tightened the tiny screw. I then tried to mount a barrel band close to the breech, but the band pulled the breech up. I sanded the band with a dowel wrapped in sandpaper till the breech didn't pop up when the band was installed. When I tightened the rear breech screws, the breech tilted enough to open a gap, but I could pull the front of the breech down by tightening the upper set screw. This kind of bent the barrel down, putting some squeeze on the forward barrel band. Things were looking OK for a fill and a half,with around 1/2" groups @ 45 Yds, then the POI shifted left and the groups started spreading way out. I have tried shimming the air tube and action with thin card stock to float the air tube, but that makes things worse. This is of course getting a mite old by now-I'm tempted to part the sucker out, but I know it'll shoot, in between poi shifts, I can do 3/8" groups at 45 Yds, then shoot the push pins that hold the target!
Here's the band in front of the breech:
[linked image]
Here's the action showing the front band:
[linked image]
The front band is tight to the air tube and the barrel is wrapped with a soda can shim all the way around. The upper set screw is just touching the barrel to eliminate play but not all the way tight.
I sure could use some advice on where to go from here, this is getting old!!!
thanks,
John

 
 Respond to this message   
AuthorReply
Daniel
(Login rdsail)
YF12

Are you using a TKO - Muzzle brake

May 6 2013, 7:07 AM 

I ask this because I have had issues with this myself. I did find out that one of the factors is the TKO. I'm extremely careful when I fill the gun not to touch the TKO. I you move the position of the TKO ever so slightly, Your POI will change.

I also added o-rings around each barrel band and that has helped a lot. I'm getting a new stock and I week wondering if I should move the back band near the breach.

[linked image]

 
 Respond to this message   
JohnL57
(Login JohnL57)
YF

No TKO or muzzle weight..

May 6 2013, 9:45 AM 

I took the muzzle weight off a long time ago as it wasn't straight and rubbed on the fill cap. I couldn't use the TKO because as yours did it would move the barrel every fill. The frustrating part is that I've seen the shift happen from one very tight group to a scatter in different spot.
John

 
 Respond to this message   
Mach-1
(Login Mach-1)
AR&P

Free float that barrel

May 6 2013, 2:35 PM 

Remove the barrel bands. Test it. If that solves the problem then you just need to keep if free floating. Try it.

My hunting 22 cal Discovery is free floating. Only use one barrel band at the end. But the band has been drilled out to 1/2". So it is just there to keep the barrel from getting bent. The barrel does not actually touch the band at all.

If free floating doesn't fix it you could have a barrel issue - small imperfections inside the barrel or on the crown.

My 22 cal did that POI shift thing too. But the shifts were only very small amounts in my case. Like you though the shifts were sudden and not predictable or explainable. It would drive tacks for 50 shots and then suddenly open up and then settle down to driving tacks again but in a different spot.

Then finally the barrel must have broken in and that POI has not moved at all since then. As a major bonus it stopped being pressure picky. Went from 1750 - 1500 psi sweet spot to 2,100 - 1,100 psi sweet spot. I did polish the snot out of the barrel with bore paste... just maybe not enough.

My second Discovery is 177. I polished the snot out of it and then polished it again... and then again before shooting it. It is a tack driver and has not shifted POI one time in the thousands of shots it has seen.

So if you're having this problem consider going back to the bore paste and polishing it again. Test and repeat as needed until you fix it. It is a bit of work... but you'll pull out less hair.

 
 Respond to this message   
JohnL57
(Login JohnL57)
YF

Pretty sure crown is ok...

May 6 2013, 10:59 PM 

It passed the Q-tip test anyway, and it does group sometimes.

 
 Respond to this message   
robert
(Login gubb33ps)
YF

Re: Benji Discovery/Katana POI shifts.. still not there..now what?

May 6 2013, 6:24 PM 

Worth it to try free floating. Even though Ive had no luck keeping POI stable, enough people have written about it that I have to believe its worth a try. I kept getting major shifts between range sessions, but I dont store or transport my airguns like they were the Dead Sea scrolls.

The two ban method you use, with the inletted rear band, has worked for me. Even during hunts where the rifle will ride along in a Jeep rear seat over country paths then get bounced along in a flat bottom boat to get where the critters are.

You do have the option of moving the front barrel band to different locations, hoping to find a spot that agrees with whatever barrel vibrations are happening. I found several spots that actually make the gun worse, but on mine having the front band near the end of the air tube worked out best.

Worth farting around with the stock. Loosen that screw ΒΌ turn and see if the POI changes drastically. Being orgainic, wood can warp/bow/bend a little, which could set up some stress. Alternately, with just one stock screw, it could shift a bit. If too loose. If loosening and tightening the stock screw moves the POI all over the place, consider bedding the stock.

 
 Respond to this message   
robert
(Login gubb33ps)
YF

some pixs might help

May 6 2013, 9:14 PM 

Stock bedding TEST:

photo 4f59de59-0f0b-46b4-b07c-02f1f7788b82.jpg

1. Loosen or remove the one stock screw.
2. Try rocking/rotating the metal in the stock. If it moves, probably will be under stress when the stock screw is tight.
3. IDEAL would be for the metal work to drop into the stock freely, but have no movement when wiggled. The stock screw just serves to keep the two together.

MINE FAILED THIS TEST! With the stock screw loose, the metal would rock in the stock.
BEDDING:

There arent great choices. What wed want was a pad of bedding ahead of and behnd the stock screw, but there arent any great choices for behind the screw. The little ledge behind the gauge is just too small.

So as much as I do not like bedding to the trigger group, in this case its about all we have.

photo cdb68d41-af27-4894-85a8-b08692f6f0de.jpg

photo 464cb90b-5f30-4e77-a5a3-1a2f3974a747.jpg


Used an inletted rear band. Split the sight base and put the band between the two sections (looks neater this waypeople have asked if it is a custom band that fits over the sight base).
photo 52f237f5-8712-475d-b585-689be5a17820.jpg]

Used a 6-screw front band. The bottom 3 screws are tight to the air tube to stop all motion. The top 3 screws are just barely NOT touching the barrel. Barrel is free to expand or contract, but has no real side-to-side or up-n-down slack.

photo c484844d-5a09-4009-af62-91572886699e.jpg

Been sitting in the safe, being moved around while other guns got worked on/shot for the last 5 or 6 weeks. Usually store at the bottom pressure. Filled it up and shot 6 5-shot groups at 20yards (same pellets/same distance it was put away shooting).

photo e135d74c-2b45-4802-9779-9c48fdc21dea.jpg

Didnt really move off POA.

Yeah...there is a good bit of DIY in a Disco. But once you get them straightened out, are great little guns.



    
This message has been edited by gubb33ps on May 6, 2013 9:18 PM


 
 Respond to this message   
Mach-1
(Login Mach-1)
AR&P

Both my Discovery Rifles Group Like That

May 7 2013, 1:27 PM 

Unfortunately I can't. Good shooting.

 
 Respond to this message   
robert
(Login gubb33ps)
YF

Get the "human" out of it as far as you can.

May 7 2013, 6:28 PM 

When going for 20 or 30 shots, tend to reduce the human element as much as I can.

Have a sister-in-law who is a sculptor. Borrowed two big blocks of scupltor-clay. Wrapped those in plastic (they will dry out). Wrapeed the plastic wrapped blocks in cloth. It's not like paly-doe, this stuff is stiff, but you can beat/pound, shape it to a pretty good custom fit for a front and rear rest. Think it tends to dampen vibrations....but that's minor compared to the custom fit.

You are still at the controls, so it's not a machine rest...but it's about as steady and "dead" a rest as i can come up with. Bet using a couple of blocks of duct-seal (being smaller, might need them on a platform) could work out just as well.


    
This message has been edited by gubb33ps on May 7, 2013 6:34 PM


 
 Respond to this message   
Mach-1
(Login Mach-1)
AR&P

Bench rest testing

May 15 2013, 1:20 AM 

Yep using bags on a bench mine will shoot about as good as yours. But without the aid of bags... not so much.

 
 Respond to this message   
Alan
(Login alpal87)
YF12

Front barrel band locked?

May 11 2013, 3:32 PM 

Do you have the front barrel band locking the barrel down as well? So far i have mine free floating. I'm not the steadiest so i rest the gun on a bipod which i stick just behind the front fill cap of the gun way out on the end of the air tube.

Have you seen an advantage to not just using the stock imbedded barrel clamp but also the clamp out front?

 
 Respond to this message   
JohnL57
(Login JohnL57)
YF

So far my best success has been with the single band mounted just behind the fill cap...

May 11 2013, 4:42 PM 

I'm kinda sick of messing with it for now, might try the poly icemaker tubing instead of transfer port later. I just got tired of taking it apart and reassembling,sighting in and burning through tons of time, fills and pellets to see if there's any improvement. I was hoping to extend my effective range to get my smarter ground squirrels rather than becoming an expert on disassembly!
John

 
 Respond to this message   
Daniel
(Login rdsail)
YF12

Just finished my disco. Sort off.

May 14 2013, 8:14 PM 

[linked image]

Here is my disco. I moved the band as you did. the front band is just for protection. the set screw is not putting pressure on the barrel but i did add an oring there.

I have not tested accuracy because I was porting the gauge port and I messed it up. so I'm waiting for a new prod gauge port from cowman.

here is a group from 40 yards
[linked image]

This is how it was shooting before I took it apart. 2100 psi to 1500psi
[linked image]

or at a lower power setting
[linked image]

 
 Respond to this message   
Alan
(Login alpal87)
YF12

robert's advice works!

May 11 2013, 3:27 PM 

I followed some of robert's posts from a few years ago on all his testing and transporting of his disco for POI shifting.
Free floating the barrel does work. I have one barrel band back at the gauge, and one other that is drilled out up at the front just to protect the barrel from getting tweaked. This benji is accurate. I find i get a very consistent 12-15 shot string out of my .22 disco and fill pressure changes depending on pellet used. If all else fails than it could just be a bad barrel, which could always be remedied with some bore paste and polishing.

I have a .177 barrel right now that has burs up at the bore and a somewhat tighter bore than the rest of the barrel. I'm using wood pencil for some and a lead plug which i can pull through into the bore but not quite all the way out and jb bore past to smooth it all out.

Edit: I absolutely love the lightness and handling of the disco. the .22 is a great little hunter, i plan on doing power mods next. In the mean time i'm building a .177 version for field target as i'm pretty confident in the accuracy of my .22, i'm trying to do the same with the .177.

Also new barrels are pretty cheap...$30 i think


    
This message has been edited by alpal87 on May 11, 2013 3:29 PM


 
 Respond to this message   
JohnL57
(Login JohnL57)
YF

It's not as if the barrel won't group-the POI keeps shifting...

May 11 2013, 4:49 PM 

In between POI shifts I can do 1/2" groups at 45 yds. then shoot the pushpins holding my target, so it seems that the barrel/crown should be ok(I believe). I did paste the bore during the last teardown, by the way-didn't seem to make any difference. I have plenty of barrel bands, so I'll try the free float next time I tear down.I'm taking a break to pop some ground squirrels with my trusty R9 .22 for now, will take another try at it when I have some more patience and time.
John

 
 Respond to this message   
Alan
(Login alpal87)
YF12

that's a great grouping, poi shift sucks though

May 11 2013, 6:00 PM 

As i said, only way i was ever able to fix it wasn't to exactly FREE float the barrel as the single set screw and single breech screw just aren't enough to keep that breech still. I physically took wood clamps to it before screwing the breech screw on, once i got that in tight, i put the barrel band back as far as i could, which was aboout 1/2" in front of the gauge and clamped the barrel down and effectively the front of the breech as well. Just like in roberts picture i put a peice of the rear sight rail barrel sleeve behind the barrel band and into the breech. after tightinging that barrel band back then i removed the clamps from around the breech and the air tube. i'm actually attempting all of this right now on my .177. Sadly i had to lap the barrel. Sounds like you're barrel is just fine. Maybe scope? or scope mounts?

 
 Respond to this message   
Darrin Simon
(Login AirheadGunner)
YF12

Mine didn't like 2 barrel bands

May 12 2013, 9:07 AM 

And it actually made things worse. You have the front one wrapped all the way around the barrel with a soda can, that may be your problem. Try the drilled band you have pictured there alone at the stock position or an inch or 2 in front of the stock position. Tight to the airtube but barely snug to the barrel, or even leave the screws loose on the barrel, free floating. These are a couple of easy things to try that don't require much disassembly. You can do it, figure it out, that's half the fun!

 
 Respond to this message   
Alan
(Login alpal87)
YF12

Re: Mine didn't like 2 barrel bands

May 12 2013, 2:27 PM 

That's pretty much what i did. One clamped barrel band far back by the air gauge and one farther up towards the front but only clamped to the air tube using 3 screws. I filed out the upper barrel hole so as it's just a thin barrel band to keep the barrel from getting bent if bumped. all i can say is it's a ragged hole shooter as long as i do my part and pick the right pellets.

 
 Respond to this message   
Joe Dzekevich
(Login JoeDzek)
YF12

Also had POI shift Disco .22

May 26 2013, 10:38 AM 

I also had the dreaded POI shift. :^) I sent the .22 Disco to TKO22 for a tune-up. Before doing the tune-up, he found that the breech screws were loose. He tightened them and then did the tune-up. Now my .22 Disco is solid as a rock. I then added the TKO22 muzzle brake and it was clipping. Gave it a 1/4 turn one way - no change. Then went 1/2 turn the other way (1/4 from the original position), and that fixed the pellet clipping. Things have been great since. Using a Centerpoint 3-9x50 scope, it is sighted-in to hit a penny sized group at 33 yards using Benjamin 15.9g hollow points. It's shooting around 815 fps and it's hitting at 22-23 fpe. Very pleased overall. You can see what TKO22 does by visiting their web site. Money well spent.


    
This message has been edited by JoeDzek on May 26, 2013 10:39 AM


 
 Respond to this message   
 
  << Previous Topic | Next Topic >>Return to Index  

D I S C L A I M E R
JK's Airgun Forums, moderators, and administrators are not responsible for any problems that may occur from reading or using content posted on this forum, as they are the exclusive responsibility of two parties: the person who posted it and the person who acted on said posted information.Use of our forums from people under 18 years old, only with legal guardian(s) present.