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Atypical Disco shot string...

June 28 2008 at 9:40 PM

Jeff P  (Login pad617)
YFOT

I have a .177 Discovery with a 2260 hammer spring in it and a power adjuster. I am using cpl?s and am trying to set the rifle up to shoot at about 800-820fps.

My best string is this:

1650 psi fill w/ power adjuster 2 turns out

1- 791.5
2- 783.8
3- 781.3
4- 800.2
5- 793.2
6- 797.0
7- 796.2
8- 792.2
9- error shot
10- error shot
11- error shot
12- 809.3
13- 798.7
14- 804.6
15- 800.1
16- 805.0
17- 806.9
18- 816.8
19- 808.5
20- 792.3
21- 802.7
22- 809.5
23- 807.5
24- 825.5
25- 811.5
26- 826.6
27- 814.6
28- 840.9
29- 840.7
30- 829.8
31- 832.0
32- 828.7
33- 840.8

As you can see I had three chrony errors but they occurred at a consistent point of the string so I am not worried about them. It looks like I am ok from shot 4-23. This is about 24.6 fps spread and about a 802.3 fps average not including the error shots. I also used some poorly shaped discarded cpl?s that have a ridge along their head so pellet quality may also be a factor.

My question is that I am not getting your typical chrony strings. In typical I mean a string that starts out a little slower than desired velocity then hits desired velocity for a few good shots and then velocity drops off. Instead no matter how I set the power adjuster or fill pressure, I start off at a slow velocity and gradually increase velocity and then at some point I dramatically increase velocity. I never encounter the decrease at the end of the fill as is typical. Is this due to the combination of lighter hammer spring and factor hammer weight? Can anyone tell me why I am seeing this?

Jeff P


    
This message has been edited by pad617 on Jun 28, 2008 9:40 PM


 
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AuthorReply

(Login jebudite)
YFOT

Jeff did you do your mods or did you buy it moded?

June 28 2008, 11:03 PM 

I see that several offer them already moded.

 
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Jeff P
(Login pad617)
YFOT

Joe...its a Mac1 prepped

June 29 2008, 12:45 AM 

I had Tim put the lighter spring in it. He adds the adjuster and recrowns the barrel.

Jeff P

 
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(Login clasicalgas)
YFOT

keep shooting,you'll see a drop off in velocity at some point.

June 29 2008, 12:12 AM 

Based on the numbers you posted,you should try a fill to about 1550,you'll see a shorter upslope.(start out at a higher velocity)I think your light striker spring needs to be compensated for somewhere if you want to flatten the curve.A debouncer would do it,and/or a lighter striker.Use trustworthy pellets if you want representative numbers.

 
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(Login gubb33ps)
YF

Looks like....

June 29 2008, 6:10 PM 

..a weaker spring and a full weight hammer. That usually brings the "sweet spot" down in pressure, but doesn't do a whole lot for consistency.

I also haven't had any luck getting a long shot count and good consistency (under 20fps spread) without using a lightened hammer. As issued, can get a decently long shot count by backing the spring tension off, but the consistency sucks (like 40fps spreads) and there is a narrow window of good fill pressure (something like 1600PSI to 900PSI). Crank the spring in (with the stock spring and hammer) get some nice high numbers (like 24-25 foot pounds in .22), but the shot count is short and towards the upper end of pressure (like 1900 - 1100PSI).

I settled on the stock spring, a little more than standard tension, and a stock weight hammer...running 21FPE+ (in .22) with a nice 26shot run with less than 20FPS variation.

 
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Jeff P
(Login pad617)
YFOT

Robert...

June 29 2008, 8:49 PM 

Is it possible to get 820+/- fps with cpl's with the stock spring and very little tension from the adjuster, or is it not possible? I guess I am asking for a 20 fpe rifle to do 12 fpe. I may try to reduce the hammer wieght. Where, and how much material would one take off of the hammer?

Regards
Jeff P

 
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(Login gubb33ps)
YF

Re: Robert...

June 29 2008, 10:00 PM 

Going to have to come off the sides of the rear section....the pin that the bolt hits to cock it keeps it from rotating but the front section should be left alone. I'd order a spare hammer before taking that step (they aren't expensive).

Might be several ways to knock the power down to 12 foot pounds...expect Crosman is working on that for the UK market.

Steve supplied a couple of nice charts and a picture of a radically ground on hammer. http://www.network54.com/Forum/79537/message/1214749516/You+need+either+a+%28much%29+lighter+hammer%2C+or+a+lower+fill+pressure.

I decided that this rifle was going to be a hunter (gray squirrels). Not that I don't practice with it, but am content with the 26 shot "sweet spot" @ 22 foot pounds on air.

For the last few weeks have been running it as a bulk fill co2, which puts it in the 11-12 foot pound class (but a whole lot of shots).


 
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(Login pad617)
YFOT

I'm going to order another hammer...

June 29 2008, 11:53 PM 

and see how much I can take off of it.

Thanks
Jeff P

 
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