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PostPosted: Sun May 19, 2019 7:56 am 
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Joined: Fri Nov 23, 2007 3:51 pm
Posts: 37
Location: Montreal
Hi everyone

OK so my newly acquired QB78 Synthetic was shooting great. like cloverleaves at 7 yards (basement) with the right pellets.

I then took the stock off and decided to polish the trigger... reassembled the gun.
.. groupings went from 1/8" to 5/8" at close range.

things I did :

I probably dry fired the gun too fast to get ride of the pressure. ( please what is the rule for degassing this gun? how slow should one go ?)

the trigger assembly had surface rust. the gun itself also has traces of lite rusting ....

I tried 2 different scopes just to be sure... but no ...same problem

I cleaned the barrel. and yes at first the patch were coming out brownish, aka traces or rust

I played with the stock screw tension and barrel screw but it did not help

what would you do to the gun now ?

also to get the best accuracy possible, how long between the shoots would you wait ?

thanks
stef


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PostPosted: Sun May 19, 2019 8:51 am 
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Joined: Fri Jan 22, 2010 1:17 pm
Posts: 1812
Location: Kingston, ON
Chrony results would tell you how fast to fire, but generally 30s to 1 min between shots should be ample time.
You know the gun is capable of 1/8" groups, and that something changed between diassembly, polish and reassembly. Check all screws, nuts, setscrews for tightness, try again. If still groups badly, disassemble and check the valve seals, breach seal and bolt o-ring - as well as the 3 barrel O rings and the barrel fixing setscrew. A very good mod for the QB is to add a second barrel fixing setscrew (or 2 new ones at +-45 degrees to secure the barrel in the breach. Finally, a metal barrel band can be beneficial over factory plastic ones.
I suspect you have a loose-is barrel. Easy enough to disassemble and tighten the setscrew.

And degassing wouldn't affrect accuracy unless you froze and wrecked a seal. I don't think you could fire fast enough to do that, speciall not this thime of year.

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PostPosted: Wed May 22, 2019 1:51 pm 
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Joined: Mon Mar 27, 2017 7:06 pm
Posts: 840
Location: Meaford, Ont.
Perhaps barrel needs leaded up. I never clean my barrels anymore until they shoot poorly. And at that it is my last resort. CHEERS!


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PostPosted: Fri Jun 14, 2019 6:45 pm 
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Location: Vancouver BC
My QB78D has a loose barrel dispite having a metal barrel band. with the stock barrel band screw it's easy to wiggle it left and right.

To tighten it first I tried adding a second breech set screw. however I was unable to get the second screw hole perfectly aligned on the center of the breach so when tightened the set screw pushes the barrel off to one side. So this technique didn't work for me and I ended up lock tithing the new set screw in place not touching the barrel.

I had better luck adressing the barrel wobble at the band by upgrading the screw to a 6/32. With the band on the barrel and air tube I threaded through a 6/32 tap. It cut some threads into the barrel at the knotch for the band screw but had plenty of clearance above the air tube. An 8/32 bolt would probably contact the air tube and that would be bad.

I use a nut driver to tighten the nuts and I have to "zero" the barrel with some test shots. With the scope windage zeroed I shoot then lossen and tweek the barrel band until the POI lines up. This way the barrel is aligned to the breech and scope and there is minimal windage shift at different ranges.

It a pain to tweek the barrel alignment this way but my QB shoots straight from 15 to 50 meters. well I got this gun from another foreign member the barrel was bent and I spent a lot of time straightening it and getting it to shoot straight at all ranges.

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PostPosted: Fri Jun 14, 2019 7:27 pm 
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Location: United States
I would assume the barrel is moving around. I would put the barrel in a vice, padded of course, then look thru the scope at whatever and gently move the gun around and see what happens. My guess is you won't like what you see but at least you can use that to figure out what thing(s) exactly are the prob and work on fixing it. I'd also move it by the scope so you can be certain it's now somehow moving. If it were me I'd put the receiver in the vise for the scope test.
Next is parallax. I'll assume you don't have an AO scope since most people don't. If so then parallax is no doubt a problem. You can adjust a non-AO scope, it's just harder to do. I can get into that if you need...
Outside that stuff I suppose I would blame the ammo. Imo people should only use good stuff, like JSB and H&N b/c they're more accurate and consistent. Sometimes people have a good shooter, like you, then it goes to hell for a mystery reason. Turns out they switched pellets or bought the same pells but really the only thing that's the same is the box. Cheap pells can change drastically from batch to batch, possibly including the actual manufacturer. Like some other mfg had a lower bid so the seller switches but of course keep the same packaging b/c they don't want you to know. If it were me I'd use JSB 8.44gr.
As for barrel leading; I don't believe that it helps. Or maybe I've just never seen one where it did, but I can't imagine how any barrel could improve with leading. So I clean them and clean rather often. The rust shouldn't hurt it, assuming the normal brownish patch and not rust that tears the patch. Normal rust powder is normal and not much you can do about it, and since it is just a little powder it's no biggie. This is the main reason i clean the bore as often as I do b/c I try to clean/oil when I'm done playing b/c it may be weeks, months or even years b4 I shoot it again. So my final patch is thick with motor oil to protect it.
So let us know about the pells you are/were using. If you don't have JSB I suggest at least trying a tin of 8.44gr. I'd also suggest the JSB 10.34 and H&N Baracuda Match but those are a bit heavy for a <500fps gun. That's just my opinion tho, but even with the more arched trajectory they should be even more accurate. They make other weights but I think the 8.44gr would be an ideal all-around choice. Maybe others who have tried a variety of pells can chime in.


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PostPosted: Fri Jun 14, 2019 9:02 pm 
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Joined: Thu May 04, 2017 6:41 pm
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Location: Vancouver BC
Adjustable objective AO scopes are pretty much the norm these days.

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PostPosted: Sat Jun 15, 2019 10:41 am 
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Location: Vancouver BC
Here's a couple of pics of the barrel band mod. One thing I forget to mention is that the 6/32 bolt gets bent when tightened. It's because the bolt head and the nuts on the other side are forced to flatten against the bands angled sides. This make the bolt single use only and has to be replaced with a fresh bolt each time the rifle is disassembled. When fully tightened and the bolt is bent I can still see a gap between the bolt and air tube.

This holds tight better than the stock bolt but a hit to the barrel or just rough treatment of the rifle will still move the barrel out of alignment. I'm thinking of adding a second band to improve this.

Attachment:
IMG_20190615_083817.jpg
IMG_20190615_083817.jpg [ 432.2 KiB | Viewed 324 times ]


Just need to Dremel cut off the extra length.

Attachment:
IMG_20190615_083733.jpg
IMG_20190615_083733.jpg [ 404.7 KiB | Viewed 324 times ]

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PostPosted: Sat Jun 15, 2019 8:22 pm 
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Location: nvvan
Do you have another scope to test it with. Maybe your scope is shot?

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