MP5K Low Power

[strykerles]

Brought a boneyard MP5K

 

gutted it

 

Titan

16:1 gears

M115 spring

SHS compression parts

3/4 cylinder

6.03 inner barrel

maple leaf 60 and omega nub

 

Air seal is good, sounds nice

Fires lovely although the power is only around 150fps with a .20

 

I'm a bit stumped tbh, granted it's the first gun I've worked on with such a small barrel

 

Any ideas?

 

thanks

[Sneaky]

You say the air seal is good? Have you checked the nozzle to hop unit sealing, when the gearbox cycles? Piston head o ring able to flex and is not brittle.

[Rogerborg]

47 minutes ago, strykerles said:

SHS compression parts

 

Including the nozzle?  Did you measure it up against the stock one?  I ask because the hop units are rather fussy, and as we know a fraction of a mm is all it takes.

 

For reference I'm seeing a full 0.95J ("320fps") out of my Galaxy MP5K with (I think) a 6.01mm barrel, M100 spring, and either the stock cylinder or a 3/4, I don't recall offhand, but not a full one either way.  Stock piston and nozzle.

 

Absolutely stock with the sloppybore barrel and whatever spring it came with it wasn't far off that at 0.9J / ~310fps or so.  There's no particular reason it should be as low as yours other than air seal.

[strykerles]

yeah the nozzle too, it's the same size as the one that came off. I think I have slightly longer and shorter ones to try

[Skara]

If compression parts seal well then your problem might lie in the nozzle/hop rubber seal.

As others have said, check your nozzle, if it's shorter than the stock or if it's overall shorter: make sure it's fully extended into the hop rubber, grab a torch and shine it down the muzzle.

If you see light coming from the hop rubber lips, it's too short.

 

Also please don't use metal nozzles. Actually don't use metal compression parts at all!

[strykerles]

the nozzle is the same size as the original. i'll experiment with some others. thanks

[Rogerborg]

Oh, now that I think about it, I hit a problem with the aftermarket barrel and Maple Leaf hop rubber not going deep enough into the hop unit.  It was a real struggle to get it in deep enough to get the C-clip on, and the barrel was straining to escape.

 

Eventually I ran a drill bit into the hop unit from the front and (by hand and very carefully) relieved it by half a mm or so.  That made all the difference, and now it's sealed like a champ.

[strykerles]

The barrel sits fine in the c clip, must be the nozzle 🤔

 

Update, slightly longer nozzle increased power to 200fps

 

and now for a slightly longer one again although it might not feed 🤞

Edited April 9, 2022 by strykerles