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 Post subject: Improving Disco accuracy
PostPosted: Wed Sep 28, 2016 5:46 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
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Location: Tecumseh, Ontario in South Western Ontario
The stock accuracy was OK but now that I upped the power I can't find a pellet that shoots well. I tinkered with the valve and port openings and I have an adjustable spring and hammer. It's shooting in the mid 900s with 14gr .22 pellets. I believe the culprit is the thin barrel. The distance between the barrel and the air tube is too small to reinforce it with a carbon or steel sleeve. Is there a higher receiver that would allow that mod and would it be worth it?
Ed.

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PostPosted: Wed Sep 28, 2016 6:43 pm 
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Location: Vancouver
I'd suggest adding another barrel band somewhere near the middle, to eliminate most or all barrel whip during the shot. You could try shooting groups with the second band in various positions.


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PostPosted: Wed Sep 28, 2016 7:22 pm 
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Location: Tecumseh, Ontario in South Western Ontario
Interesting. From what I've read though the air tube is not the most reliable anchor. Apparently as the pressure changes and possibly the temp it can affect the barrel's POI. Actually I tried anchoring the barrel to the air tube near the muzzle with a spacer and electrical tape and I couldn't get a descent zero on the scope. Perhaps a solid band would work better.
Thanks,
Ed.

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PostPosted: Wed Sep 28, 2016 7:39 pm 
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Location: Coalmont BC
You could try the "six screw mod." on the barrel band.... There is one setscrew on the bottom now (6 o'clock).... Drill and tap two more at roughly 2 and 10 o'clock to the reservoir tube.... Drill and tap three additional holes in the barrel portion, at 12, 4, and 8 o'clock.... Install in the original location, using only the three setscrews in the lower hole, tightening them against the reservoir, so that the barrel is centered in the upper hole in the barrel band but not touching it.... Grind the cup points off the three setscrews for the upper part, so they don't mar or bite into the barrel.... and then snug them up so that they JUST touch the barrel, preventing vertical or side-to-side movement, but allowing it to react to temperature changes or pressure changes in the reservoir.... You may need a small drop of blue Loctite on each upper setscrew to prevent them coming loose....

I found that worked really well.... some have found it works better in a different location than stock.... Experiment with it and you may find a significant improvement in accuracy and the ability to hold consistent POI....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Wed Sep 28, 2016 7:44 pm 
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Location: Vancouver
That's why I suggest trying different locations. It could be that a second band just a few inches behind the first would stiffen it enough, without getting into the middle of the air tube where the slow shrinkage through the usable pressure range might not be significant. Something else to consider would be the type of bands with O-rings embedded in over-sized holes. Those apparently lend stability without being so susceptible to influences like a 0.0005" dimensional change in an air tube during the course of dozens of shots. I've not had a Disco, so only guessing. But with my HiPAC'd Crosman carbine I found no vertical shift between 3000psi and 850psi, regardless of where I put the barrel band. I'd expect a HiPAC tube to flex a bit through that much pressure range, but even as it sits now with a 14" unshrouded skinny Crosman barrel and a Crosman barrel band there's no change through the fill.

Not suggesting it isn't a factor to consider of course. Perhaps you could epoxy on a good quality carbon tube, then once it's cited file and sand away just the bottom face until it slips over the cylinder easily. A paper thickness of air between CF sleeve and cylinder ought to be enough.


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PostPosted: Wed Sep 28, 2016 8:11 pm 
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Location: Ontario
you might try jsb 15.9 or 18.1 if you are shooting that fast. They shoot best out of my disco in mid 800 fps and many others say jsb best pellet for a disco.


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PostPosted: Thu Sep 29, 2016 5:03 am 
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Location: Central West River Nova Scotia
From my own experience, I found that pellet speed and pellet choice, are not necessarily optional. At the new power level, you may find a pellet that likes that speed, but I have found that chasing down the right pellet, IE, size, weight, shape, skirt, and brand, to be a frustrating experience, when all I really needed was a change in speed. Barrel twist rates are only forgiving within a certain range. During an extensive test period a while back, I found some guns, don't like slow pellet speed, and some don't like fast, no matter what pellet. Further to that statement, I have even shot Crosman HP,s with extreme accuracy when I found the right speed.

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PostPosted: Thu Sep 29, 2016 6:52 am 
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Joined: Mon Aug 24, 2015 4:56 pm
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Location: Tecumseh, Ontario in South Western Ontario
A lot of good advice, thanks.
I've ordered another barrel band and will do the 6 screw mod on both of them. I won't be disappointed any further if it doesn't work because I've learned airgun accuracy is no simple or singular matter. Velocity is one of many factors and I'm in the process of analysing my many pellets (including the Jsb 15.9 and 18.1gr) at different speeds.
We are happy when we find a pellet that works and p'd off with all the other pellets we bought that are now obsolete. :lol:
Ed.

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PostPosted: Thu Sep 29, 2016 10:03 am 
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The trick there is to buy more airguns. I've just this past week bought 2,500 JSB RS 13.43gr pellets as I wanted to test a higher velocity, flatter trajectory pellet in my HFT rifle. Sure enough they work nicely in the QB78D... but they also turn out to deliver nice groups in my old 2260, QB57, and my son's 2240. Making my RWS Hobby tins obsolete. Except... they work best in my Webley pistols! Nice to have fall-back airguns with which I can feed the leftovers. And the couple of thousand JSB Exact Heavy 18.13gr still sitting around? They're fine, as I'll still use those in the QB78D for squirrels.


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PostPosted: Sun Oct 09, 2016 8:46 am 
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Joined: Mon Aug 24, 2015 4:56 pm
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Location: Tecumseh, Ontario in South Western Ontario
I decided to try the 6 screw mod on the existing barrel band. First I had to enlarge the opening for the barrel to be free floating. I carefully tightened the screws to just touch the barrel using a light from behind to guide me. The barrel is so long and thin it would easily bend before my fingers noticed any tension on the Allen wrench. I let the Loctite set overnight and tried the gun. The poor results left me scratching my head. It must of been the screw tension that was causing the erratic groups. I tried moving the barrel in the band but it was solid. I loosened the top screw a quarter turn and I could hear some clicking when I tried moving the barrel. The results were no better but it might of been because the barrel was still touching the 2 lower screws. So I tried loosening them but it might of been too much because the groups were the worst yet. The key was getting the screws adjusted just right and it might of also been because I didn't grind the end of the screw flat like Bob suggested. Anyway I thought I'd use a method I tried on another gun. I removed the set screws from the barrel portion of the band and used the handy holes to squeeze in silicone. I let it set and bingo!
Many people use the term tack driver but I thought I'd prove it.

Image

Image

It took 3 attempts at the 5 tack sets from 12.5 yds. but it was a lot of fun. First I tried wadcutters thinking they'd do a better job of hammering but the Crosman FT pellets are more accurate in this gun.

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PostPosted: Sun Oct 09, 2016 10:43 am 
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Location: Coalmont BC
Glad you got it sorted.... I often use O-rings in my barrel bands, but there is no room for one in the Disco between the barrel and air tube.... Good solution with the Silicone !!!!

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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