2260 Basic (revised for hot temperatures)

2260 Basic (revised for hot temperatures)

Joined: November 17th, 2006, 3:51 am

June 26th, 2011, 10:35 pm #1


Revisiing the 2260 Basic.

Got a bit less basic over the months between then and now, but its mostly external add-ons. Left it set up for 12gr use, getting 32-35 good shots per 12gr., and still producing about 11.4 foot pounds per shot.
http://www.network54.com/Forum/275684/m ... Basic+post

BUT I had tuned it up back in March, when temps were in the low to middle 70's.

High temperature use (it's been in the 90's for weeks) showed a need for a bit more smack to the valve to get back up to speed. Velocity was down nearly 85fps on the first shot, climbing with each shot as the gun cooled down a bit.

Bringing the gun back into the house to cool off into the middle 70's, and the speed went back up to where it was last set (about 600fps).


Heading towards valve lock. Gas is producing higher pressure due to the temperatures, that pressure its holding the valve closed harder, so need a stronger striker smack to the valve stem to get it open.

Got busy making a half shroud for the gun, mostly because it looks a bit less bulbous that the original LDC, partly for improved dust collection (pictures of previous version in the above linked posts).

New System:
[/IMG]

To cure the slow speeds in hot weather, could just put on a "power adjuster" end cap, which allows increasing or decreasing striker spring tension. Could just shim the spring forward to increase tension, but thought I'd go at it backwards.
[/IMG]

With the new guide placed INSIDE the striker, the weight of the guide moves back and forth with the striker, adding weight and energy to the hit to the valve. Rather than shimming, the thickness of the guide's head pre-tensions the striker spring.

And it acts like a cocking indicator.
[/IMG]

Honestly had to remove the guide and shorten the head of it a touch, as it was first made was giving a little more smack to the valve than I wanted (speed up/shot count way down). With a little less pretension, got it back to about the original speeds/shot count (about 600fps/14.3gr/11.4 foot pounds).



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Joined: November 28th, 2002, 6:26 pm

June 27th, 2011, 1:02 am #2

...that adding mass to the striker, while of course it does increase valve duration and gas release, also moves the sweetspot of the velocity-vs-temperature curve toward lower, rather than higher temperatures.

http://www.network54.com/Forum/79537/me ... 3fps+stdev



Steve
Last edited by pneuguy on June 27th, 2011, 1:52 am, edited 1 time in total.
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Joined: November 17th, 2006, 3:51 am

June 27th, 2011, 2:55 am #3

..and had been using the mod shown as a cocking indicator (although originally using delrin, so the weight wasn't the original intent).

Tried the mod in brass to give a PCP version a little more weight in order to get it to run at higher pressures. Could have just used more spring, but past a certain point, having to yank on the bolt to cock a gun seems crude.

Will have to cool the gun down and test it at lower temperatures to see how it behaves. Shop AC cranked up, a few frozen gel-pacs, and a stick on thermometer should do it.
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Joined: November 28th, 2002, 6:26 pm

June 27th, 2011, 1:07 pm #4

...are prettymuch the only variables you have to work with to increase striker momentum, and there comes a point where more spring makes cocking a chore.

Steve
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Joined: November 17th, 2006, 3:51 am

June 28th, 2011, 12:48 am #5

Did try the cool down test, and it surprised me by showing nearly no difference in velocity from 92 degrees to 72 degrees. 20Fps should b e worth something, but in this case it seems to be worth nothing

Cranked the shop AC up, closed the door, and got the gun (and the shop) down to 72 F in about 2 hours..

Stock 24 barrel, recrowned and cleaned. Not too shabby for a cheap co2 rifle and a $30 scope (including mounts). Took 4 shots to get it roughly resighted in, and were a couple of on targets shots after shooting the 25 shots on paper, so call it 30 shots per 12gr./ 14.3gr./604fps/ 11.6 foot pounds per shot for 30 shots when shot at 72 degrees.

Got about the same speed last time out (5 fps SLOWER), at 92F, but more like 35 shots per 12gr. There is enough variation in gas weight in 12gr. to show that much difference, so really can't claim the high temperature shot string to be longer...call it even.

And it can shoot pretty well for a cheap co2 gun (have to look for previous post on 2260 Basic to see what was done to it).
[/IMG]

This is a freak group... have gotten occasional groups about this small, but this one is notable because it is pretty well centered right on the POA.
[/IMG]

Can't count on that...but do count on the cheap little rifle putting a pellet on a grasshopper at 20-25yards.
Last edited by gubb33ps on June 28th, 2011, 12:49 am, edited 1 time in total.
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Joined: November 28th, 2002, 6:26 pm

June 28th, 2011, 12:53 am #6



Steve
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Joined: November 17th, 2006, 3:51 am

June 28th, 2011, 3:16 am #7

At least with the Crosman system, its starting to look like the 12gr. is acting like a mechanical stop for the valve. It has to hit the 12gr. in order to pierce it, but the piercing stem is shaped to not allow it to keep piercing deeper. So no matter how hard you smack the valve (momentum type of smack), it won't open farther.

Valve return can be slowed by the weight of the striker and the spring tension on the striker when all the way forward (call that post tension?).
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Joined: November 28th, 2002, 6:26 pm

June 28th, 2011, 12:56 pm #8

...by shortening the piercing pin?

Steve
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Joined: July 6th, 2005, 5:26 am

June 28th, 2011, 2:49 pm #9

my thoughts are to shorten the valve stem. you will not get any more lift but you will get more impact velocity. Myself I prefer bulk over 12 gr. so I have modified breach, hammer and bolts to allow for a longer stroke on my 2260. I also use the cocking indicator too. I have tried the pistol grip and plastic butt stock and have gone back to the much better wood stock and much better trigger of the rifle. John
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Joined: November 28th, 2002, 6:26 pm

June 28th, 2011, 3:58 pm #10

...because it allows a longer hammer stroke and therefore more hammer kinetic energy for any given hammer spring force. The length of the valve stem is seldom what determines lift -- hammer energy + valve seat reslience + valve closure spring force versus pressure do.

Mainly, a long valve stem is a safety/reliability feature, because it limits the likelihood of (in the case of 22xx style power plants) the cocking pin striking the end of the bolt.

That's why, in 22xx's, if you shorten the stem you should also consider shortening the bolt.

Steve
Last edited by pneuguy on June 28th, 2011, 3:59 pm, edited 1 time in total.
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