Calvin's VM Build Thread

[Calvin]

In the mid-OOs, VMs were a passion for me. At that time I had access to a milling machine, so I did lots of wacky things to them and collected a hoard of parts. I never explored all the things I wanted to with them and sold all that stuff a few years after I stopped playing. When I started up again last year, I told myself I was cured. I was a new man. I would never go back to the old ways. Well, the old ways were stronger than me.

I purchased a couple VMs from a member and received them a week ago. My goal was to get one running. Been at it a week, and figured I would start documenting what I have been through for the next poor bastard that suffers my affliction.

[Calvin]

The older VM was in the exact sorry state that I remember as being typical. The newer one was in pretty good shape, with some old paint in a few spots. The torch came out right quick.





I expect to put a double trigger shoe on, so I needed to rescue to trigger group from the older one, which was in sorry shape. I also planned on using the parts from the old one, and the body from the new one. I placed the nicer bits from the newer one aside so I have a layer of bits to fail at if necessary.

[Calvin]

My first order of business was to get a feed system that works. I sat with the original feed block, my calipers, and some beers for an evening. I have settled on a feed neck for other builds that has proven rock solid reliable, so I incorporated it into sort of vert right side feed. Its a very short ball path and placed the hopper reasonably low. I had to design a new set of magic fingers, so I tweaked them a bit to be a smidge closer together to center this smaller paint properly in the feed. It only takes 4 minutes to print, so I was able to move through prototypes quickly until it was just right. I also made the fingers a bit thinner to get the suppleness I wanted. They are printed in PEBA 90A. All in all, this part of the project went pretty easily. Throughout testing so far, the resulting printed block has felt remarkably solid. I am going to make a spare with a strengthening rib running vertically through the mount screws. If this one ever fails on the field, I can just mount the beefier one and go.

[Calvin]

The next order of business was to get her holding gas and cycling again. I placed the valve on my sizing chart, and it told me that 016 was the proper size. Although this was true, I found that it didn't like the ones from my Captain Oring's Player's kit. Instead, it preferred the very slightly thinner buna 016 rings from Kleindorfers. They are an Ace affiliate, so any Ace should have those.



I replaced the ASA oring with a buna from Kleindorfers as well. Forgot the exact size, but it is often referenced and easily found.

That left me with the dreaded hammer seal to replace. I had been mulling this problem for awhile, and felt that I could print a 90A Peba seal that would work. They are very tough, and have similar suppleness to urethane 90. First I created a torus with the proper ID and OD measurements as my calipers read them. Then I made a cut on the top and bottom of the torus to create the flats.



I printed and adjusted a few times to get the output from the printer to match the measurements from the original. Finally, I had to very carefully clean the print with a fresh xacto and sweeps of the torch to cleanup hairs and stringiness. In the end, I succeeded. Felt like an incredible triumph. I probably have 500 cycles so far in it, and have pulled out the hammer to inspect it often while working on other mods. I don't see any wear or issues with it so far.

[Calvin]

Now that it was cycling, it was time to work up a bolt. I have made bolts before for Spyders, and the loss of mass from the bolt/hammer system lost me too much velocity. Given the extreme mass of a VM hammer, I was confident that I could make what I wanted and get it functional. I settled on a 3 oring design using 014 buna orings. Testing with the first showed me that I could have a long nose on the bolt and still have the front oring 3mm part the lip of the feed for sealing purposed. This let me put a nice ramp at a shallow 9 degree angle. I made the bolt 2.6mm longer than stock, and could probably add another .7mm to be flush with the back feed lip. Right now it's working great, so I'm going with it.







It cycles well. Was shooting 240-260fps with the stock barrel. Feeding has been great so far.

[Calvin]

One of the things I had wanted to do back in the day was the AGD spring mod. The stock springs can achieve orbital velocity on the unwary. At some point, I wanted to be able to have much of the upper tube milled off, so getting that spring out was a must. I now have many AGD springs from my Mag adventures. I printed up a buffer/spring guide from Peba, and put in a longish silver one. Worked just fine, and shot about 5fps less than stock. I think I can easily do spring tension adjustments by altering the shape of the buffer/guide to compress the spring more while maintaining the same overall length of the buffer to match the white nylon stock buffer.

[Calvin]

I have a freak back on the way from Doc, so I know I have will 20fps or so available from tighter barrel sizing. My hope is to get enough overhead, that I can use an HPR to adjust velocity. I have been doing all testing on a 825PSI output tank. Since I needed to get more velocity out of the system, I pulled the valve out again to do all the old tricks I remember. I drilled and tapped the three useless ports to 8-32 and locktited in set screws. I drilled out the main port to match the passage from the valve to the bolt. Then I bored out the valve entrance a smidge, taking care to leave the sealing lip wide enough to not give issues.



I also inserted lighter, slightly longer spring.

These changes brought up velocity to 265-285fps. I was getting in the ballpark now.

[Calvin]

The final issue was the valve stem. Amazingly, both stock stems were holding air and functional. However, I needed to mod up a thinner stem for more airflow into the bolt. I didn't want to cut up the originals. After pondering all the various types of stems I had, I stumbled onto a bag of brand new Tippmann valve stems that I have in a bag. They have a very thin stem right where I need it, and a wider mid portion that I realized could be handy in modding what I needed, The seal itself seems just right for my valve. I picked up 5/32 brass tubing and some 1/8 steel wire. After cutting the tubing to a proper length, I bored it out just a tiny bit, and then pressed it onto the Tippmann stem with my vice. I cut a length of the steel wire and JB Welded it into the tubing butted securely to the original steel Tippmann stem. Once it cured, I cut it to the proper length, and then cleaned it up and polished it on the drill press. I gave it about 1/32" longer length than the original Sheridan valve stem for a tiny extra bit of dwell. I also removed the 3 ears that Tippmann has on the head of their valve stem.







After reassembly, the VM is now shooting 270 - 290fps through the stock brass barrel. That should be just right after I get a properly sized freak barrel on there. Once that arrives, I will decide on whether spring tension will adjust velocity, or if I have created enough velocity overhead that I can use a regulator instead.

[Calvin]

I also needed to get that old trigger group completely cleaned. It got degreased, all pins polished, and put back together. It's not that hard to reassemble if you take your time.

[Calvin]

Once the barrel back gets here, and the system is finalized, I plan on adding an old Impulse trigger frame along with a double trigger shoe. Should be a great shooter once that all gets completed.

[Chaos]

Hopefully that seal for the hammer holds out for you, I'd be interested to see the results. Otherwise I recommend doing the hammer mod I came up with because once you do that you won't need that funky seal anymore and can use a standard off the shelf o-ring. And it's nice to see that my impulse frame mod is being put to good use 👍

[Calvin]

I'm curious to science how long this seal will last. Been trying to find someone local to open up the slot for a 113 without success. If I don't succeed by mid winter, I will send both hammers off to one of the MCB airsmiths to have it done.

[Calvin]

I have been working on this build since last posted, and I figure its time for some updates. I did some experimentation with bolt designs. During testing of those I had 3 ball breaks. Two of them were discovered as I took the hopper off. There was a ball in the chamber that had a hole pecked into the back, and the paint had oozed out. Clearly a clipped last ball in the stack. The last was when I was trying out a bolt with the same air hole as stock, but a horn shaped ending that cupped the ball (that sounds wrong). It was very clearly blown to smithereens right as the valve opened, as the mess started 1" into the barrel, and it flew out in a mist.

I started looking at the feed area, and realized I couldn't see into the feed block enough to know how the balls were feeding. I made a new block without the 45 bend and feed neck. This immediately showed me how much room the ball had to forwards and backwards with the stockish magic fingers I was using. I went back to the drawing board and made a new magic finger design, followed by several variations to get the spacing perfect. After putting the feed block back on, I sat for awhile with the LTR on and pushing paint. I had no air and no spring in, and manually cycled many balls through while watching. What I saw was that the pressure on the feed stack could randomly push balls upwards a bit into the square area of the feed cut in the body. This allowed the next ball in the stack to project out into the breech to be chopped. I went back into cyberworld, and added a shape into the feedneck that filled this space and forced the balls straight down into the breech.



I added the strengthening rib to the feedblock while I was at it, and made the new version. All feeding issues had ceased. In testing all changes since then, I have had no pinch ball breaks of any kind. I really like this addition because it adds a mechanical lock inside the breach that greatly strengthens the feed block when mounted.

[Calvin]

After this, I went back to bolt mods. I found some things that I liked and some I didn't. I gradually settled on a "wide mouthed" version with a flat 5 degree anti-chopping ramp on the nose in front of the first o-ring.





None of these bolts moved the velocity way up or down. I managed to snag another of the newer black VMs off the bay for a cheap price. This freed me up to do some body mods that I had been avoiding. I used a D-HSS bit to enlarge the path from the bolt tube down into the valve out to 6.25mm diameter.



I then tapped the top plug hole in the body out to 5/16-24 and locktited in a short set screw there.
Finally, I used the same D-HSS bit to enlarge the passage from the volume chamber under the ASA into the valve spring chamber. After deburring and cleanup, I reassembled and tested. HOLEE COW! At 825psi, I was shooting 365 very consistently. I slapped a Sidewinder on and dialed it back till it stopped recocking, then back up an quarter turn. When I added paint back, it was still shooting 330psi. It was time to lighten springs.

[Calvin]

I fabbed up a new set of TPU buffer/spring guide parts for the back of the bolt and front of the rear block. They were setup for the stock feed springs. I had some hardware store springs of the same diameter, but settled on a 3 1/4" piece of my shittier stock spring as perfect.







The marker cycled very smoothly and the trigger feels great with this spring. The lowest I could get with my wide mouthed bolt and a good no-paint recocking margin was 305fps. I went back to my stash of bolt experiments, and put the most constricted one in. Perfect! I was easily able to chrono it at 280-290. After I degassed, I threw the reg onto a pressure gauge to find that it was shooting that speed at 490psi. Not too shabby!



My plan now is to remake my wide mouth bolt design with a constriction on the entrance hole and 90 bend, leading out into the widemouth chamber. I will vary that constriction size until I have a nice range of adjustability that has me shooting field velocity around 525 psi. That should put me 100 psi or so over no-ball recocking issues, and yield a reliable marker. Once I have a modern barrel on it (Help me Obi-Docinobi. You're my only hope!) I will probably have to adjust that constriction to be a bit smaller still.

All this has shown me that the whole system has to be opened up from the volume chamber to the bolt entrance before you can really start reducing operating pressure. Even though that wasn't my actual goal, it pleases me to be able to run a much lighter main spring now. The trigger already feels waaaay better. With a double trigger shoe and some work to limit over travel, I will have a decent rate of fire. The recoil is also much more mellow now. I may try and find someone with a lathe to lighten my spare hammer. I think the system would allow it now.