GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: AirCats on October 21, 2015, 11:44:54 PM
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I know this is no longer a new model and much had been said about it, hence the "revisit"....
While I was on the Natchez site (buying them $6.99 Horton scopes), http://www.gatewaytoairguns.org/GTA/index.php?topic=97768.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=97768.0) I noticed they have refurb MK-177 for $34, and adding it to the order did not increase the shipping by much..... I have a M4-177 and while it is fun to play with, its shells don't go the full length so is quite fragile as a pump rifle, it is much more suitable for tethered HPA/CO2 use (converting it right now).
Now the shells on MK-177 goes the full length therefore is a much more robust design for a pump gun, and I still want a low-end tactical pump for unknown reason. and the $34 price tag was hard to resist. So I ordered one.
( the $6.99 Horton scope from the same order fits the Mk-177 very nicely.)
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I have high hopes for this little rifle.
the plan:
* open it up and check everything over. Probably have to recrown the barrel on this one (the crown is off-center! maybe that was why it got returned in the first place).
* put everything back together, run through chrono for fps at 4, 6, 8 pumps.
* modify bolt (extend the probe; add o-ring), run chrono again to see if fps increase.
* base line accuracy test.
* add barrel/pump-tube shim and then lock them together using pinch-type hose clamps (so the barrel and pump tube becomes one rigid unit).
* add epoxy putty to fill the gap between shells and the pump tube, for a tighter fit.
Then test accuracy.
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I do not know how durable they are at 10 pumps and less I had to se what the pump and valve were capable of and it did not stand up to that...
The good news is it makes an excellent 20.5 fpe MK1322 carbine...
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took it apart (lots of screws).
first, the bad: the shells are full length but not nearly as thick as I thought they would be. The M4-177's shell is a lot thicker (too bad it is not full length).
now, the interesting...... this is not a dressed-up 760:
-- the pump tube is much larger. The tube in the M4 is 5/8" ID, the tube in this MK is 3/4" ID (same as a 2240 tube). The piston and valve are therefore also larger. Valve is aluminum.
-- striker weights 47 grams (2240 striker weights about 65 grams if I remembered right). The M4's striker weights 30 grams.
-- striker spring is much stiffer. Stiffer than both the M4's and the 2240's.
I start to suspect the power plant is from a larger pump rifle.....
(in the photos: MK stuff on top, M4 stuff below)
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The Mk177 power plant is unique it has the 760 stroke with a larger diam p-tube/valve... I got mine to 905 fps with 7.9g CPHP 1024 when I accidently cheated...
http://www.gatewaytoairguns.org/GTA/index.php?topic=79303.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=79303.0)
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I just picked one up from someone on Craigslist the other day that had no business shooting it... no logic... he said there was "a" pellet stuck in the barrel, I tried pushing it out from the front and it didn't budge so I took the thing apart, I could see one at the breech, one in the transfer port... pressed the barrel block off and found there were 4 pellets stuck there...
Anyway, my findings as they kinda negate a couple of your planned mods:
-The barrel block is pretty beefy compared to the M4 and Powermaster 66 unit
-The transfer port seal is built into the barrel block which seems pretty nice compared to the normal square O ring type seal
-There seems to be more rubber molded to the barrel block, as it seems like the probe has drag that feels like rubber on plastic not plastic on plastic
I kinda like it, but it's a bit flimsy, I might mod one of the two I have into something more fun :)
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yeah, just noticed that the MK's barrel support already have somekind of rubber seal installed, much better than the M4's no-seal/open-gap. Nice! no o-ring needs to be added on the bolt. The bolt probe can still benefit from an extension, it will seat the pellets deeper allowing full rifling engagement.
The MK piston area is about 50% larger than the M4's, and the stroke is about 2x as long, this thing can probably generate 3x as much power. Might be too much for the plastic bolt.
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The pumper linkage on the M4 is the same as the Powermaster 66, so it has a longer stroke than the MK which is the same as the 13XX and 760...
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hmm, you are right. I was excited by the much longer push-rod in the MK, didn't realize that doesn't mean the piston will have longer travel.
Looks like the MK starts at about 6" and ends at 9.75", travel=3.75"; M4 starts at 4.25" and ends at 8.75", travel=4.5". So even with the much larger piston area, the MK is only moving 20% more air per pump.....
M4 on top, Mk below
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valves: length about the same; MK valve has larger OD, larger ID and deeper cavity; By rough estimate the MK valve probably has 3cc of internal volume; M4 valve volume is about half of that.
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both are able to push the pump cup all the way to the valve ....
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scrapped the original mod plan, and ordered parts to convert the MK into tethered PCP.
Will retain the pumping mechanism, this will allow the possibility of going ACP (air conserving pump) or tPCP (tethered pre-charged pneumatic) or PPPH (precharged pneumatic pump hybrid).
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I have been wondering, can the MK valve be flat topped? If it could, it might fit in a benji 392.
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I have been wondering, can the MK valve be flat topped? If it could, it might fit in a benji 392.
Probably not. The nose cavity goes almost all the way to the tip of the nose.
what is the reason behind flat topping a valve? In stock config the nose cone conform to the pump cup cavity, there doesn't seem to be a gap.
I thought the 392 valve is larger?
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Flat topping makes for more pressure capability. With the normal pump cup setup you can only get so much performance. I have a Crosman Recruit that I flat topped, pinned the valve and put a pin in the piston face and a few other things that at 10 pumps you get about 750fps with a Crosman pointed pellet, if you pump 15 times you get a shot at around 750fps and one at around 500fps... I don't think you could get there with a pump cup
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scrapped the original mod plan, and ordered parts to convert the MK into tethered PCP.
Will retain the pumping mechanism, this will allow the possibility of going ACP (air conserving pump) or tPCP (tethered pre-charged pneumatic) or PPPH (precharged pneumatic pump hybrid).
Ended up putting the MK177 project on permanent hold.
I bought and received the 1/16" NPT adaptor and tap for the project, so far so good. But..... when drilling the valve, the hole ended up being 0.030" oversized. The drill bit size was correct, it turned out that the impressive looking floor-standing drill press I was using has a damaged chuck :'(.....should have just used my little HarborFreight bench-top press. :-[
Crosman's web site doesn't show a parts diagram for the MK, there is no way to get a replacement. Although the whole thing (refurb'ed MK) is only $34, I don't want to get another one just for the valve.
.... no more MK project.
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project is up again. Just bought a new one from sportsmansguide ( $50....get free shipping and a $20 gift-certificate... so in the end about the same as what I paid for the refurb one ).
Mod/test plan this time around:
1) test fps of stock configuration.
2) test fps with extended bolt probe.
3) flat-top the valve and piston (it turns out to be possible, just not as straight forward as the 1377/760 valves), test fps.
4) maybe drill the valve and install the 1/16" adaptor to convert to tethered HPA. This will depend on the kind of fps achieved from step #3 (if flat-top result is not that great, I will leave the valve alone and just add a pressure tube ahead of the valve and attach the tether there. The pump tube will not be pressurized).
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got the new one and get the fps in stock config (indoor range, about 70F; 5000 ft elevation)
CPHP
499, 500 @ 6 pump;
546 @ 8 pump;
577, 580 @ 10 pump
speed is consistant, but even at 10 pumps it is only 6fpe.
I really don't like pumping this thing. The plastic shell has too many sharp edges. The M4-177 is not fun to pump either but not nearly as unpleasant as the MK.
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installed long-probe bolt, the fps increased by about 40fps :
with CPHP,
537/533 @ 6 pumps
585 @ 8 pumps
622/619 @ 10 pumps
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MK-177 tethered HPA / pump hybrid experiment.......http://www.gatewaytoairguns.org/GTA/index.php?topic=102349.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=102349.0)
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That is a very interesting build, and I might do something similar as I've found myself with 2 MK 177's :). I'm tempted to do something like the millennium pumper and add an onboard reservoir, and by that I'm thinking of adding a CO2 canister as the reservoir, trying to make it removable so it can be used on CO2 as well... But that project is for another day when I actually have time to do anything...
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It doesn't look like people are having much luck with this gun in stock form. Even with a bigger piston, crosman still can't make power.
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my 417
(http://i746.photobucket.com/albums/xx102/robnewyork50/fairly%20complete.jpg) (http://s746.photobucket.com/user/robnewyork50/media/fairly%20complete.jpg.html)
fun guns one theyre converted (-'
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Rob M... i had a rifle just like yours when i was in the Army... every time i stroked the Co2 canister something down range blew up... lol
I love it man, That looks great. go all the way and make your Co2 canister look like a M203 grenade launcher...
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lol, a few people said to go all steampunk on it.. But yes, i think rap 4 sells an airsoft m203 launcher that may conceal the massive tank on the front,.