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PostPosted: Tue Jul 17, 2018 11:02 pm 
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Location: Vancouver BC
My barrel is bent but does that mean the bore is too? Don't suppose they make them this way?
Thinking about putting the barrel under the car wheel and prying up the bent end. That's how we fix bent bicycle forks. How soft/springy is this metal?




Background story...
I received a pre-owned QB78 and right off I though the barrel was bent downward a couple inches forward of the barrel band.
Could have happened in shipping or maybe that's typical of QB78's? I don't know.

I have it stripped down and the barrel on a known flat surface and it rocks back and forth positioned on its 'top edge'. When positioned on its bottom edge (upright) there is a gap under the barrel slightly off center closer toward the barrel band. So it is definitely not straight on the outside.

I've had the gun for a couple of weeks and it shoots inconsistently throwing pellet low, then high then grouping in a single hole for two or three shots then back to scattered groups. I've shot it at 10 and 15 meters on a sturdy bench rest and with the rifles bi-pod. At 10m groups range from 0.20" to 1" over 42 shots (one powerlet). At 15m it forms two or three separate groups. My best at 25 meters had me land 7 shots out of 42 on a 45mm field target. I'm testing indoors and the temp is very consistent at 19-20 deg Celsius. The rifle has a 4-16x40AO scope and I'm at 16mag when bench shooting.

I've taken it all apart looking for clues that would equal a gas flow velocity problems like the valve filter but this QB78 seems to have no gunk buildup inside and the basic mods done, like...

> Rear piercer removed for one powerlet operation. I get 45 shots before POI drops
> Trimmed hammer spring. The sale description say 'tuned to the max with trimmed hammer spring to bring it in under 500fps'.
> Two washers behind hammer around the base of the spring. These must be the 'hammer shims' I'm guessing although I don't yet understand their purpose.
> Water line tubing transfer seal (crudely cut).
> Filter bit removed from inside valve.
> There is a oring seal on the tip of the bolt probe.
> Seals look good except for the barrel seals which I probably damaged pulling the barrel out.

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PostPosted: Tue Jul 17, 2018 11:14 pm 
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Maybe I'm expecting too much from a QB78 but I think it should do better than this.

Sample groups, 10m, 5 shot groups.
Shots 11-15 look good and are on target one mil dot over the cross hairs on the 10 dot.
But there are repeating patterns such as...

Pattern A: shots 1-5, 16-20 and 36-40
Pattern B: 6-10, 21-25, 26-30,

Shots 41-45 are just scattered.
Velocity drops at shot 46.


Attachments:
QB78-5SHOTGROUPS.jpg
QB78-5SHOTGROUPS.jpg [ 657.6 KiB | Viewed 792 times ]

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PostPosted: Wed Jul 18, 2018 12:14 am 
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Location: Coalmont BC
Only the last inch of a barrel is important.... Check out this FWB Target Pistol owned by Dr. Robert Beeman....

Image

A slight bend to a barrel means nothing, IMO....

Bob

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PostPosted: Wed Jul 18, 2018 12:15 am 
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I shot this '1 shot per target' at 10m on the bench with bi-pod. Crosman HPP .22
I think it shows the wondering point of impact better than five shot groups.
Crosshairs on the 10 dot. QB78 tuned sub-500fps

Comments?

Shot #2 is on target-1 by accident. Target-2 left blank.
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1shot-2.jpg [ 691.67 KiB | Viewed 776 times ]


Attachment:
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Last edited by Pushtone on Wed Jul 18, 2018 12:22 am, edited 1 time in total.

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PostPosted: Wed Jul 18, 2018 12:20 am 
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:shock: :? :lol:

OK, then I guess I CAN bend it back to as close to straight as I can it and I won't mess it up any more.

rsterne wrote:
Only the last inch of a barrel is important.... Check out this FWB Target Pistol owned by Dr. Robert Beeman....

A slight bend to a barrel means nothing, IMO....

Bob

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PostPosted: Wed Jul 18, 2018 12:27 am 
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Where is your bipod mounted?.... Is there anything pushing on the barrel in an inconsistent manner?.... What does the crown look like?....

One thing I do with QBs is add two more mounting screws to the receiver on the sides of the loading tray, pressing on the sides of the barrel at the very back....

Image

You can see them in the above photo.... The QB barrel isn't mounted very solidly in the receiver, with a short overlap and only one setscrew.... The addition of the two extra screws, tightened against slight flats on the back sides of the loading tray area really stiffens up the barrel to receiver joint....

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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PostPosted: Wed Jul 18, 2018 12:43 am 
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Look what I found inside the valve...

1. With no spring guide the valve spring is always off center. I just realized this is where the .22LR casing goes as a guide.

2. The washer under the valve spring has be cut away (to increase flow maybe) but the openings are asymmetrical and different sizes.

Could either one or both be the cause of my wondering POI?
I can also see the valves transfer exhaust has been enlarged and a bit of an angle ground out at the pin side of the hole.


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Attachment:
IMG_0985.jpg
IMG_0985.jpg [ 85.06 KiB | Viewed 768 times ]

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Last edited by Pushtone on Wed Jul 18, 2018 1:13 am, edited 1 time in total.

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PostPosted: Wed Jul 18, 2018 1:11 am 
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The last visible mod I found are these two washers around the hammer spring guide.
Shims?
What would be the effect of removing them or adding two more?
Attachment:
IMG_0986.jpg
IMG_0986.jpg [ 224.23 KiB | Viewed 767 times ]

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PostPosted: Wed Jul 18, 2018 1:23 am 
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rsterne wrote:
Where is your bipod mounted?.... Is there anything pushing on the barrel in an inconsistent manner?.... What does the crown look like?....
Bob


Hi Bob,

The bipod mounts to the sling stud on the forestock. Like this
Attachment:
xs78-right.jpg
xs78-right.jpg [ 35.46 KiB | Viewed 763 times ]


Don't have any experience evaluating crowns but it feels smooth to my finger and when I drag a cotton ball over it no strands get caught, so....

I've cleaned the barrel with a 3/16" dowel and dry paper towel patches after I shot about 300 old tarnished pellets and the accuracy got worse.
Since cleaning I've shot 750 new CHPP pellets.

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PostPosted: Wed Jul 18, 2018 2:06 am 
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I haven't had much luck with the premier h/p's in any gun I've tried them in.
Maybe trying a different pellet might show improvement.
Out of the 4 or 5 QB bbls I've been through, one turned out to be a stumper.
It was a .22, and came on an A/R 20-78. I tried everything with it, and could not get it to group. They do ship the occasional bad bbl, but I haven't heard much on this.
When I'm shooting on C0-2, I try to space each shot from 90-120 seconds. It seems to prevent excessive cool down of the tube....
Try some proven quality pellets first- JSB comes to mind here. My good Q-B/A-R bbls have a preference for the JSB heavy in .22...

-D.S.

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PostPosted: Wed Jul 18, 2018 10:06 am 
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I missed the fact you haven't tried a variety of pellets.... Airguns are fussy critters, that is the first place to start, IMO.... Get some 14.3 gr. JSB Exact Express and some of the 15.9 gr. Exact Jumbos.... even some of the 13.4 gr. Exact RS.... I would be shocked if one of those three doesn't improve your groups over the Crosman HPs....

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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PostPosted: Wed Jul 18, 2018 11:20 am 
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OK I ordered two tins of JSB from D&L.
I read that QB78 love crosman pellets and the previous owner used CHHP with good results.
I was hoping a QB78 would hit a 45mm field target kill zone at 30-40meter.
I'll get a chance at those distances at the end of the month as I plan to take the rifle to a HFT meet but to do that I feel it has to be more accurate at 10m.


Q: do the washers (shims?) around the hammer spring add power or reduce it?

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PostPosted: Wed Jul 18, 2018 5:47 pm 
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They add power.... Install too many and you won't be able to cock the gun....

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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PostPosted: Sat Jul 21, 2018 9:26 pm 
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Good news! My QB78 likes JSB 15.89 grain, hates anything lighter.
I started with the JSB (Cometa) Exact Jumbo RS 13.43 gr. and got random scatter shot.
Then onto the JSB Exact Jumbo Express 14.35 and same thing, scatter.
At this point the Crosmans HP were performing much better. The CHPP would at least occasionally group with randomness in between good groups.

I was composing the rifles Craigslist ad in my head when I loaded the JSB Exact Jumbo 15.89 and... Holly Heck, one hole groups.
Very good repeatable groups. Now trigger control and hold mean something!

I had no idea it could be so night and day. I'll get some heavier pellets next and see if this is a one pellet rifle or just like the heavy ones.

Thx for the help


Check out the difference:
JSB 14.35 gr.
Attachment:
IMG_1012.jpg
IMG_1012.jpg [ 398.67 KiB | Viewed 571 times ]



JSB 15.89 gr.
Attachment:
IMG_1011.jpg
IMG_1011.jpg [ 417.06 KiB | Viewed 571 times ]



QB78D - modded valve, extended probe, one powerlet, stock hammer spring, no shimms,
4-16x40 scope at 16mag
Bench rest
10m
20 deg C
3-shot groups


Again here the full shot sequence for the 15.89 gr. Getting 27 shots on one powerlet. Now I can take it to sunday dinner at the folks and try 27meters next.
Attachment:
IMG_1008.jpg
IMG_1008.jpg [ 395.79 KiB | Viewed 571 times ]

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PostPosted: Sat Jul 21, 2018 9:39 pm 
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Nice! That's a big difference =).
Glad you found the right pellet. My Qb78 loves the 15.89 too, though it also loves air arms/jsb 14.3gr even more. Every gun is different though.

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