I receive many emails regarding the PMI 3 Vm-68 series paintball guns due to the number of parts we sell for them.  I bought and sold PMI 3s and Vm-68s when they were first released.  I also used them as rentals at my field for many years.  I won't pretend to know everything (as many people do), but I can tell you I have a quite a bit of experience and insight into the VMs and the common problems that might arise.  I'll list a few of the major ones here in the hopes of saving you some time and saving my email in box some space.

Problem 1:  Continuous cycling. (Does not re-cock, just continues to cycle, sounds like a full auto.)  This is the most common problem the Vm has. 

Solution 1: The original PMI 3 had a cocking slot on the side of the body that was too long. It allowed too much of the blow back gas to escape before the lower bolt reset.  This was the major reason for releasing the VM-68 version, it had an updated body with a slot almost 1.5 inches shorter.  If you have the original PMI 3, there is not really a repair for this problem,(originally, you could have sent your PMI 3 in for a VM-68 body update). Now days, your gun is worth more as a collectors piece than a working model.

Solution 2: The VM-68 has a four position valve body.  Turning this valve clockwise with the VM-68 valve tool can increase or decrease gas flow by changing the diameter of the hole the gas flows through.  Using the largest hole (which is what I recommend) allows the most flow to fire the ball and re-cock the system. The largest hole should be in the 12:00 O'clock position. The largest hole is usually marked with a red or black mark or a small metal detent.  If you have to adjust velocity, use the upper bolt adjustment, not the valve.

Solution 3: The VM-68 still had some blow back issues, even after the body update and valve position adjustment. But these problems can be avoided with some knowledge.  The most common cause is to much drag on the upper bolt.  This can be caused by several things.  Paint and dirt in the body, not enough lubricant, or worn/swollen upper bolt o-rings.  To prevent this, keep the gun clean and lubricated, inspect the bolt o rings by looking longways down the bolt.  The o rings should very close to flush with the side of the bolt itself.  The bolt should move smoothly with very little resistance in the gun body.

Solution 4: Still having problems ? The next step is to replace the lower bolt front o-ring to capture more blow back gas to improve the re-cock cycle.  If that does not work, replace (if you can find a new one) or stretch the upper bolt main spring.

Solution 5: This is a little extreme, but it's what we did with our rentals and it worked great. This trick works to allow the valve to stay open longer, allowing more gas per shot to cycle the gun. Remove the valve and re-assemble the gun without the Exhaust valve cup seal spring.  The cup seal will rattle a little when the gun is not gassed, but it will stay in place.  It will seal once the gun is gassed up.  When the gun is fired in this manner, the valve stays open much longer, allowing more than enough gas through the system to fire and re-cock (even in extremely cold temperatures.). You will get less shots per tank, but your blow back problems days are over. 

Solution 6:  There were several aftermarket upgrades, like the magna-port valve, and pro-formance lower bolt kit.  If you can get your hands on these items, they do make a big difference in performance. 

Problem 2:  Multiple ball feeds. (Feeds and shoots more than one ball per cycle).

Solution:  This is an easy one, and applies to most paintball guns.  Most guns have "Ball Detent" which only allows one ball to chamber per cycle.  On the VM-68 these were a pair of orange or black fingers attached to the feed port.  Many old schoolers call them "Magic Fingers".  If you are having ball feed issues, replace them to solve your problem. To extend you ball detent life, remove the two allen screws and reverse the fingers outward if you are going to store the gun for an extended period of time.  This will prevent the fingers from retaining a folded position from storage against the closed bolt.

Problem 3:  Erratic or inconsistent velocity.

Solution 1: Because the co2 source is so close to the valve on the VM-68, it was easy to occassionally get liquid co2 (yes...co2 can be a liquid for you new guys, that's why your tank reads ounces not PSI, but that's another story.) in the valve, this at worst would cause velocity spikes (into the 450's I've seen) usually resulting in a ball break in the chamber. At best your velocity would fluctuate by 10-20 FPS until the liquid expanded into a gas and settled down.  To solve this problem, Sheridan released the VM-68 Magnum (co2 tank in the rear of the gun) and later the EXC-68 (co2 tank at the bottom of the grip, with a factory expansion chamber).  To help with this problem, install a back bottle system or bottomline system with expansion chamber.

Solution 2: This problem is not as common as above, but I see it quite bit because many players don't know about it.  The VM upper bolt has 4 o-rings, the 3rd holds in place a small spring and ball bearing that prevents your velocity adjustment screw ( it has a flat spot that locks in place.) from spinning during the firing cycle.  If the adjustment screw turns while firing, your velocity will constantly be changing.  Many players changed the o-rings , but lost the spring and ball bearing in the process.  If you bought the gun used, or had it handed down, you might want to check this area out.

There are other problems the VM can have, but these are the biggies.  Overall it is a very durable, rugged, unique looking gun and that is why it has the "cult" following it does.  If the marketing guys at Sheridan were in touch with the real market, they would bring it back into production with some enhancements instead of putting out the disposable stuff they do now.

I hope this helps. I can't promise, (I've got a lot going on...) but I don't mind chatting about the VM's via email or phone if you have other questions or suggestions. 

Stay low and shoot quick !  Ron.