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

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

Joined: September 22nd, 2010, 12:29 am

May 6th, 2013, 2:14 am #1

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:

Here's the action showing the front band:

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
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Joined: December 29th, 2012, 1:22 pm

May 6th, 2013, 11:07 am #2

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.

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Joined: September 22nd, 2010, 12:29 am

May 6th, 2013, 1:45 pm #3

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
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Joined: October 29th, 2008, 2:38 pm

May 6th, 2013, 6:35 pm #4

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:

Here's the action showing the front band:

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
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.
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Joined: November 17th, 2006, 3:51 am

May 6th, 2013, 10:24 pm #5

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:

Here's the action showing the front band:

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
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 ¼ 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.
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Joined: November 17th, 2006, 3:51 am

May 7th, 2013, 1:14 am #6

Stock bedding TEST:



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.






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).
]

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.



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).



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.

Last edited by gubb33ps on May 7th, 2013, 1:18 am, edited 1 time in total.
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Joined: September 22nd, 2010, 12:29 am

May 7th, 2013, 2:59 am #7

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.
It passed the Q-tip test anyway, and it does group sometimes.
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Joined: October 29th, 2008, 2:38 pm

May 7th, 2013, 5:27 pm #8

Stock bedding TEST:



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.






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).
]

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.



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).



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.
Unfortunately I can't. Good shooting.
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Joined: November 17th, 2006, 3:51 am

May 7th, 2013, 10:28 pm #9

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.
Last edited by gubb33ps on May 7th, 2013, 10:34 pm, edited 1 time in total.
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Joined: July 29th, 2011, 1:44 am

May 11th, 2013, 7:27 pm #10

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 ¼ 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.
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
Last edited by alpal87 on May 11th, 2013, 7:29 pm, edited 1 time in total.
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