GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: Tack Driver 10 on August 16, 2020, 05:22:10 PM
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I have a few MK177 pump tubes that have aged like fine wine.
Time to pop the cork on one and savor another 22 cal Chubby build.
The MK tube is 3/4" vs 5/8" so hence the Chubby designation.
This version will have a 3 1/2" stroke equivalent to a 5" stroke 2200.
The initial build will use the stock aluminum valve that is ported with a light valve spring and reduced valve volume.
Still has over 2X the volume of a 2200 so not expecting big power.
Stock, the MK177 produces around 625 FPS with 7.9 CPHPs and 10 pumps, so not a real race horse for sure.
Hoping to get close to that in 22 cal with this version.
The pump link is a NOS 766 version that was modded to position the link pivot optimized for the 3 1/2" stroke.
Custom barrel band and piston using the MK cup seal.
Put it together for velocity testing.
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Ran the gun over the chrony this morning, pumped up real easy (easy like Sunday morning).
I was somewhat surprised as it performed better than I thought it would.
22" barrel with 14.3 CPHPs:
05 pumps= 476 FPS 07.19 FPE
07 pumps= 565 FPS 10.13 FPE
10 pumps= 657 FPS 13.70 FPE
12 pumps= 696 FPS 15.38 FPE
15 pumps= 748 FPS 17.76 FPE
Should be capable of 700 FPS with 10 pumps and 800 FPS with 15 pumps.
Took the gun apart and performed some valve mods.
We'll see what she does in the morning after a good nights rest.
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Healthy numbers ready! Can't wait to see what it does after the mods.
I still have one of these tubes and a couple valves to play with. One of these days I'll get around to it.
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Made valve mods as follows:
Trimmed the front of the valve tip .010" to better fit the cone seal
Duro 90 poly seal on the valve
Opened exhaust from .140" to .160"
Bored valve floor back .100"
Concave valve seat and stem
Decreased valve volume by 50%
Increased throat from .200" to .235"
Power increased some but the pump cup is just not up to the task.
The pump arm springs open to about 45 degrees at 15 pumps indicative of excess dead air space.
05 pumps= 519 FPS (+43) 08.55 FPE
07 pumps= 591 FPS (+26) 11.09 FPE
10 pumps= 679 FPS (+22) 14.64 FPE
12 pumps= 716 FPS (+20) 16.28 FPE
15 pumps= 760 FPS (+12) 18.34 FPE
I think I need to call in the seal expert to straighten this out.
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Time to kick the pump cup to the curb.
Made a shiny new flat top piston head and valve cap.
Everything else is the same except opened the valve volume up a bit.
We'll see how the gun performs in the morning.
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Ran the gun over the chrony with 14.3 CPHPs:
Made one additional pass at 20 pumps (because I could).
05 pumps= 514 FPS 08.39 FPE
07 pumps= 603 FPS 11.54 FPE
10 pumps= 693 FPS 15.25 FPE
12 pumps= 736 FPS 17.20 FPE
15 pumps= 783 FPS 19.46 FPE
20 pumps= 828 FPS 21.77 FPE
Previous performance level with pump cup:
05 pumps= 519 FPS (+43) 08.55 FPE
07 pumps= 591 FPS (+26) 11.09 FPE
10 pumps= 679 FPS (+22) 14.64 FPE
12 pumps= 716 FPS (+20) 16.28 FPE
15 pumps= 760 FPS (+12) 18.34 FPE
Most of the gains were made at the high pump/pressure levels as expected.
So all things considered the pump cup performed respectfully with some mods.
With a .177 cal 24" barrel should be close to 1000 FPS, 17.54 FPE using 7.9 CPHPs with 20 pumps.
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Smokin!! 8)
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Smokin!! 8)
With the current setup, the gun is a good compromise between power and ease of pumping.
I like the pump curve as is, basically an up to 10-15 pump gun depending on desired use.
I could lower the valve volume and make more low end power but would be harder to pump and limit the high end.
I'm going to put a gauge on the valve so I can monitor the pressure at various pump levels.
This tube would make a nice 160 stocked pumper which is the reason for this experiment.
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Installed a .177 cal 24" barrel on the gun just to see what it would do.
Was hoping it would hit 1000 FPS.
Came close with CPHPs so tried a few softer, heavier pellets.
The RWS Super Points came the closest but just shy of the elusive 1000 FPS mark.
Close enough for government work....LOL
10 pumps= 814 FPS 11.62 FPE
15 pumps= 908 FPS 14.46 FPE
20 pumps= 979 FPS 16.81 FPE
Gave the gun a touch more cam over.
20 pumps= 989 FPS 17.15 FPE 7.90 CPHPs
20 pumps= 998 FPS 18.14 FPE 8.20 RWS Super Point Xtra
20 pumps= 978 FPS 17.92 FPE 8.44 JSB Exact Diablo
Gauges should be here today so I'll get one on the gun and see what pressure it's developing.
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Took the gun apart (again) for gauge surgery.
Miraculously the patient made a speedy recovery.
05 pumps= 900 psi
07 pumps= NT
10 pumps= 1900 psi
12 pumps= NT
15 pumps= 2600 psi
20 pumps= 3000 psi (full dump)
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Put the 17 cal 24" barrel back on the gun for a final test 3-15 pumps.
Gun pumped a little harder perhaps since the gauge reduces the valve volume some.
Velocity increased , somewhat peppier than a hot 2100.
With CPHPs 7.9
03 pumps= 450 FPS 03.55 FPE
05 pumps= 615 FPS 06.63 FPE
07 pumps= 737 FPS 09.53 FPE 1200 psi
10 pumps= 843 FPS 12.46 FPE
12 pumps= 884 FPS 13.71 FPE 2100 psi
15 pumps= 919 FPS 14.81 FPE
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Sweet build!
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Very very nice! Wish I could do the machine work necessary for things like that. 8)
What’s the max pressure a valve modified like that could take?
3000 psi close to max?
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Very very nice! Wish I could do the machine work necessary for things like that. 8)
What’s the max pressure a valve modified like that could take?
3000 psi close to max?
I've pumped one gun to 3200 psi and not sure what the failure threshold is.
Realistically about 1000-2000 psi is a good usable pressure vs velocity.
Beyond that the gun gets increasingly more difficult to pump.
This build was an experiment using the MK tube.
Has the same swept volume as a 2100/2200 tube but this valve has a lot more volume.
Can make a multi shot pumper or more volume for a 22 cal or higher caliber gun.
The 3/4" tube fits nicely into a 160 or similar wood stock for an all wood and metal pumper like an mini 1400.
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Almost a full 22 ft. Lbs. with a 14 gr. pellet in 22 cal. In a light weight pumper like that is just too cool!
Super nice build indeed! :D 8)
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Almost a full 22 ft. Lbs. with a 14 gr. pellet in 22 cal. In a light weight pumper like that is just too cool!
Super nice build indeed! :D 8)
Right, I think it will play nice with the original Chubby.
This gun has the same MK tube but a 4" of stroke equal to 5 3/4" of a 5/8" tube so a bit more power.
Recently upgraded the pump arm with walnut to match the walnut/ red oak stock.
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some nice machining Ron
i have made a pumper from a qb78 which is very close to a 160,i used a sheridan lever on it and a longer main tube
i'll see if i have any pics of it when it was together
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some nice machining Ron
i have made a pumper from a qb78 which is very close to a 160,i used a sheridan lever on it and a longer main tube
i'll see if i have any pics of it when it was together
I'd like to see your build if you can find the pictures.
Van made a gun that I really liked similar to what I'd like to do with this gun.
Used a 2260 stock and 2200 tube with a 13xx FT valve.
It's the top gun.
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i looked but couldn't see any on this computer
i'll get the rifle out it has had the barrel and breecch taken off at present as i was getting ready to pin the pump arm onto the lever
i think from memory the inlet seal was sticking so i may try a new style of inlet
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Still loving seeing your Builds...especially the Chubbies.. ;)
Since I was sticking to a stuffed plastic piston and pump cup I went for max valve volume...about .24 ci... that with the stock 16" mk .177 barrel got me to about 905 fps with 7.9 Benji hollows at max pumps... then accidentally found a 10% rise in valve temp ( from 65*-70* f to about 130* f) got me to about 1030 fps.. ;) (about a 10% rise in fps thanks Boyle)...
I do have everything needed to build one more that is aging like wine also... Still have not decided between .177 and .25...
Now that there are .30 JSB pellets...well how about a Tack Driver .30 ;) ::) :o ?
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I'm not a big bore fan in a pump gun.
A 30 cal Chubbie does sound enticing but would require a lot pumping.
There's plenty of valve volume available to support it though.
As an alternative a heavy 22 cal pellet can get you in the ballpark.
22 cal JSB Beast pellets at 34 grains are the heaviest I've used.
In a hot 2200 Magnum with 15 pumps FPE increased from 19.81 (790 FPS) to 23.52 (559 FPS).
I like 17 cal the best with 22 cal a close 2nd as long as I can get decent velocity without a lot of pumping.
17 cal JSB Beast pellets at 16.2 grains are also out there as an alternative to 22 cal.