Museum

[mobiusmorte]

Any help is appreciated.

I have a SMG-68, that I am trying to restore to working condition.
I am having difficulty increasing the velocity, it seems that no matter what adjustments I make the velocity will just not come up, it is shooting probably around 100fps (eyeball chron)
I have used the following power sources,

Standard CO2 tank
HPA with a LP Output (350-400 psi)
HPA with a HP Output (800psi)

I have tried all 3 with and without a regulator.

To my eye I do not see a difference at all in the velocity no matter which air source is on the gun.

This SMG also has an adjustable rear cap which I am assuming can be used to adjust the velocity (Screwing it in should increase the tension on the drive spring causing it to open the valve longer when the hammer strikes correct?)

Note: I did have to fabricate a new guide pin for the drive spring, since the one that came with the gun was bent and twisted when I received it.
Replacement pin is ultra smooth stainless steel. I do not think the pin is the problem.

I have read that the SMG-68 prefers a CO2 siphon tank, which I have not been able to acquire yet, but is the pressure provided by a Siphon CO2 that much higher than the 800psi I am getting out of my HP output tank?

writing all of this out, did bring one possibility to mind, do you think it is possible that the drive spring is just too weak to drive the hammer hard enough into the valve? If so I would be highly appreciative of any sources that may be able to provide a replacement spring.

Thanks in advance for your help!

[Azzy]

Well, running liquid CO2 has to be looked at like this.

Normal tank = flash to gas, pressure of 800~ PSI, comes out valve, powers gun.
Siphon tank = Liquid comes out valve, expands a few hundred times its volume, that expansion powers gun.

try fogging the gun (hold the gun pointed down and flood the valve while dry firing, till snow comes out) and then shooting a piantball across the chrono.

[c6quad]

you really should be using a siphon tank. i'd go ahead and change the spring too. there is a guy on ebay that has parts for the smg. Jay is the guys name but is ebay name escapes my mind right now... PBJay... is part of it. maybe someone here will know who i'm talking about.

i've heard of people trying to use HPA but the internal gas line on the smg is only 1/8 macro line and is too small.

[c6quad]

here's a link to his store on ebay. he doesn't have smg parts listed but email him and tell him what you need and he'll get you fixed up. eBay Store - PB Sports Paintball: Marker Accessories, Other Paintball Items, Paintball Markers

[mobiusmorte]

Awesome info, thanks.

[Chappy]

Not sure if this applies as my issue was with a 68 special, but I'll mention it. The powertube on the special I restored(and sold ) was beat up pretty badly. And since the valve actually slams into the powertube to open up, once the face of the powertube is beaten in the valve won't open as far. Thus low fps. I couldn't get miy special over 220, even with siphon and a new spring. Once I figured out the problem and put in an nos powertube, 300+fps I think the smg uses the same powertube/valve setup, though I'm not sure. But if it is, consider the powertube a consumable item when you rebuild.

And, yeah, I agree with everyone else, use a siphon. Did you weigh that hammer? It needs all the help it can get

EDIT after some more thought

Originally Posted by mobiusmorte

This SMG also has an adjustable rear cap which I am assuming can be used to adjust the velocity (Screwing it in should increase the tension on the drive spring causing it to open the valve longer when the hammer strikes correct?)

Correct

Originally Posted by mobiusmorte

is the pressure provided by a Siphon CO2 that much higher than the 800psi I am getting out of my HP output tank?

Well, figure a normal CO2 tank runs 800 psi, and a siphon will typically do 50 to 70 fps higher than vapor.

Originally Posted by mobiusmorte

do you think it is possible that the drive spring is just too weak to drive the hammer hard enough into the valve? If so I would be highly appreciative of any sources that may be able to provide a replacement spring.

PM me, I have one I can send you.

[c6quad]

the smg doesn't have a power tube but that's a good point Chappy. instead of the valve hitting a power tube, it actually hits the magazine itself. the magazine is aluminum too so you should check it to make sure it isn't beaten' all to hell like the power tube problem Chappy described.

[HP_Lovecraft]

The rear hammer cap is not intended for FPS adjustment, but was often used for that purpose. There was at one time a replacement cap that had an adjustable dial. So you might get some extra FPS by adding some shims/spacers/nickels behind the main spring, then backing out the cap to adjust. (usefull if the spring has substantially weakened over time).

This is if you still have a problem AFTER getting a siphon tank.
There is a poorly written HOW TO INSTALL A SIPHON guide here.

Siphon tanks were an option with the SMG-60, but became standard with the SMG-68. Its tough to get SMG-68s and 68-Specials running without them.

Plus, also check the hammer/orings. The rear hammer tube was made from low-grade steel, and rusted/pitted very easily. Sometimes just a quick polish/hone of the inside tube made a huge difference.

nick

[Chappy]

Originally Posted by HP_Lovecraft

The rear hammer cap is not intended for FPS adjustment, but was often used for that purpose.

Hmm, you know, I read that as if he had an rva installed in the rear cap, but you make a good point.

[mobiusmorte]

The help available here continues to impress.
OK, the internals are in pretty good shape, not beat up at all.
I did do a pretty decent cleaning and polishing job on everything as well, will do another more thorough polishing while I am waiting for new parts to arrive.
The magazine is also in amazingly good condition.
The siphon tank is a definite must have at this point along with the new spring.

Sending you a PM about that Chappy.

P.S. I guess I really am getting old, 20+ years of paintball and I completely go brain dead on CO2 output pressure. It doesn't seem all that long ago that I switched from CO2 as my primary power source but it has been over 8 years... crapola I am getting old. Maybe all the gray in my hair really isn't really tough to waashout paint residue like I've been telling myself...

Well, all out of smilies so...