The PMI-III prototype made it's first public appearance at the 1990 Bay City Open, along with the Tippmann 68 Special, and has since become a legend in the paintball world. The Sheridan PMI-3/VM-68 is a true open bolt blow back semi automatic paintball gun and is a nearly indestructible work horse designed to last a lifetime. With around 300,000+ guns produced it has amassed quite a following through the years.
One of the great features of the VM is that it has an almost unlimited upgrade path. Nearly every component on the gun has an aftermarket part allowing you to increase performance and shed some weight. For more on upgrading your VM, the VM FAQ 6.2 is a must read or check out other people's custom VMs to get some ideas.
Amazingly, new-in-box VM-68s can still be found from time to time. Prices can range from $75-200. Your typical used, stock or mostly stock VM usually sells for around $50-90, with $75 being pretty average.
The one down side to VM's these days is that it is an old gun and upgrades are becoming harder to find. With enough persistence you can still round up even the most obscure VM upgrades though. If you're looking for VM parts, be sure to check out this list of places that still carry some items.
On the plus side, there are a number of other paintball guns that use VM-68 style barrel threads. These paintball guns include the Armotech WG-65 series, Sheridan Equalizer, Edge PG-1 Pro, Tornado, and older style Bruizers like the Pro, Pro XP, Sport, and Patrol models.
This does NOT include newer Bruizers like the Bandit, S.O.B. (TNT), Apocalypse S.O.B., Outlaw, and Fugitive. Those have Spyder threads.
For those of you that are into technical info, Punisher has informed me that, "The body, end caps, feed block, and rail are 6063 extruded aluminum. The rest is mainly low grade steel, though the front ASA is a little crumbly and is probably an alloy of steel and something else."
Why was the name changed from PMI-3 to VM-68? Well, from what I've gathered so far the name change is a direct result of Sheridan being sued into bankruptcy and then getting bought out by Crosman. Crosman did the name change, not Sheridan. Why? I don't know for sure but I assume that once Sheridan went belly up their contract with PMI was null and void. No longer needing the 'PMI' in PMI-3, they changed the name and went with National as the distributor (PMI was the original distributor).
A quick note about Sheridan Equalizers. The Equalizer was a blow-forward gun (like an AutoMag) produced in 1995. Barrels, feed blocks, and sight rails where interchangeable with VM-68s. Unfortunately the Equalizer never took off and the gun was discontinued. Now days they can be had for as little as $50.