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PostPosted: Mon May 18, 2015 3:31 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
Within reason, sure, it can be accurate. But it comes down to the twanginess of a springer coupled to the abilities of the shooter to cope with that factor. I'm getting loonie-sized groups with my QB57 and a Burris Fastfire III and figure with a LOT of practice to get my hold absolutely consistent I might be able to get that down to quarter-sized. Maybe. The 61 is doing a bit better than that with the decent 6x scope and a lot of care with every shot, but I doubt it has much better than quarter-sized potential even rested on a bag.

The barrel chopping notion comes from Chevota. He made the suggestion that the last few inches, in fact anything over 10" to 12" of my QB57 barrel were probably not necessary, even perhaps slowing the pellet. As it turned out that's not quite true in my case; I ended up ordering a fresh barrel for the QB57 after finding that 10" was costing a few too many feet per second. 14" turned out not to cost me anything (stock was about 16") in speed with 14.3gr JSB Exacts, and actually added a little velocity due to lack of extra barrel friction with 11.9gr Hobbys. It's likely that every spring piston airgun would respond differently to trimming the barrel. Re-crowning nicely helped a little with accuracy compared to the rather crude, asymmetrical crown on the factory QB57 barrels. With the MP-61 I think moderate velocity is best, focusing on smoothing out the spring (grind the ends level and polish, make a longer delrin guide which fits well while allowing free motion, very light moly lube, no spring preload other than 1/8" for the end of the spring guide) rather than trying to hit too hard. Especially for a kid. No doubt in a few years (or less) he'll want a PCP so he can hit the mark every time. But I think shooting springers was good for me as a kid and it'll be good for my boy.


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PostPosted: Tue May 19, 2015 5:52 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
More fussing with my son's rifle... good grief. The openness and sometimes seeming randomness of shots bothered me too much so I looked into barrel weights. Started by turning the ID of a piece of brass tube until it was a snug slip fit on the exposed last inch of the barrel, then soldered that into some copper pipe, then turned a delrin muzzle brake around that. A lot of press-fit stuff. Seemed to help a little but the groups still looked too open for something that's supposed to be a target rifle... time to tune the barrel.

So I made a bronze bearing weight. Haven't weighed it, but it's an easy press fit onto the carbon fibre shroud and heavy enough to make a significant difference. I started with it right at the brake, 1" back from the muzzle, but the groups were worse than without the weight. Slid it back to the middle of the barrel and there was no improvement. So I did some reading in benchrest forums, and it seems most folks go with a fairly heavy weight out near the muzzle and tune by fractions of a millimetre... so I started playing with different spacings just behind the brake. Sure enough, groups changed quite a bit from one place to the next even with very small adjustments. I'm getting about 8mm centre-to-centre at 6.5 metres consistently with the front of the weight 10mm back from the brake.

Image

Time to get back to my actual job for now, but this is promising:

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Oh, and because my boy seems to be struggling with keeping the scope above the barrel, often tilting it to the right, I carved out some aluminum from the back of the Leapers offset scope base and glued in a bubble level:

Image

He likes it, enjoying watching how custom the thing's becoming. I'm hoping that by further tests with barrel weight position I can shrink groups to 10mm at 10 metres. That'd be good enough for this one I think.


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PostPosted: Wed May 20, 2015 5:22 pm 
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Location: Victoria, BC and Clarkston, WA
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PostPosted: Wed May 20, 2015 7:15 pm 
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Joined: Tue Oct 13, 2009 1:27 am
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Location: Vancouver
Seems so. This afternoon I took a bit of time to adjust the barrel weight at 10 metres, my neighbours taking a break from smoking on the back porch below my trap... Checked a few settings and found that about 4mm back from the brake worked best in reducing group size. Here's a 10 shot rested group. 7 shots in the middle I managed to hold the little rifle properly. The 3 outside that hole were me messing up, slipping slightly as I fired because it's just such a little toy and I struggle to hold it correctly.

Image

About 6mm centre to centre for 6 of those, 11mm including the one on the left. Very acceptable. Now it's just up to Haru to shoot bullseyes.


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PostPosted: Thu Mar 10, 2016 6:10 pm 
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Joined: Thu Mar 10, 2016 6:03 pm
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hi there
im in the middle of strugle to find best scope placement. I bought Walther 4x32 DC CQB and same mount as you. Mine just does not keep dimmensions :D
It looks quite like yours and may be the same inside.
How far do you keep an eye from the scope while shooting?
For me it feels quite close when using yours setup and I'm thinking about intermount scope at the end of the barrel.
BR


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PostPosted: Thu Mar 10, 2016 9:58 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
keneida wrote:
i'm in the middle of strugle to find best scope placement. I bought Walther 4x32 DC CQB and same mount as you. Mine just does not keep dimmensions

I am not certain of your meaning with this. Do you mean that the gun/scope will not hold zero from one session to the next, or within the same session? It could be that you haven't tightened the mount thoroughly, or used the two slide-prevention set screws under the scope to bed into the plastic of the gun and keep it from moving due to recoil.
keneida wrote:
How far do you keep an eye from the scope while shooting?
For me it feels quite close when using yours setup and I'm thinking about intermount scope at the end of the barrel.

My son has been shooting the gun, not me, so eye relief is a bit different between my 54 year old eyes and his 9 year old eyes. But it's a few inches anyway. Maybe 2 inches or a bit more for him.

However, we have given up on using the IZH MP-61 for him in FT now. It's just too weak to knock things down reliably, even at close range. A very under-powered .177", giving only 3fpe at the muzzle, 2fpe at the closest targets. Barely audible *tink* when it hits steel. I've moved his scope over to a carbine modified 2240 with a 14" barrel and with the heavier pellets (he'll shoot RWS Hobby 11.9gr) travelling at around 490fps he'll have a much easier time knocking things down.


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PostPosted: Fri Mar 11, 2016 6:55 am 
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Joined: Thu Mar 10, 2016 6:03 pm
Posts: 2
GerardSamija wrote:
keneida wrote:
i'm in the middle of strugle to find best scope placement. I bought Walther 4x32 DC CQB and same mount as you. Mine just does not keep dimmensions

I am not certain of your meaning with this.

I'm sorry for bad english. I was refering to the mount. The small block which you screw to the rest of the mount is to high. So when you screw everything together makes the mount tilt left so the barrel and the scope would not be in the same vertical axis.
I could tilt it more to make it steyr aug style :D


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PostPosted: Fri Mar 11, 2016 1:02 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
That sounds like there is something wrong with your particular rifle. We had no trouble with the mount staying vertical. Perhaps it is something which could be corrected using a file and some skilled plastic removal? Treacherous work, I know, but a slanted scope mount would be very difficult to use.


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