Calvin's VM Build Thread

[Calvin]

A few more days into it now. This project is a great way to learn blowback mechanics. I have also learned that the designers of this were smart people. Opening up the air pathways through the system yielded a marker that had a very high velocity ceiling. Adjusting velocity with a regulator does not work as the range of fps adjustment is a band that isn't that wide before it ends in recock failure. My epiphany was that the regulator was for the operating pressure of the bolt system, and that you need two constriction points. Once is in the valve area to set the band of velocity range. The other is in the bolt for precise velocity adjustment. This is, in fact, exactly what the designers did. They just set it up for CO2. I changed my valve hole from a 6.35mm to a 5mm hole, which brought the range band down to a usable level. I think 4.5mm is probably just right. I have two more stock valves, and will be setting one of those up at that size for the second VM body I have. Also, I tried buna 015 instead of 016 for kicks on a valve swap. Worked just fine. Urethane 90 015 felt too tight on my body. I have found 600-650 psi to be the best operating pressure with the 5mm valve hole. The gun runs smooth at this pressure, and isn't any louder than my other blowbacks.

I went through many bolt designs (eleven to be exact), till I got to the Mk XII. I am very adept at the fail to win strategy, and definitely showed my chops on this project. I tried thin and thick oring designs. I tried making oringed and oringless copies of the stock bolts using the metal back caps and 3/8-16 adjusters. My breakthrough was to chuck up slightly oversized oringless bolts on the drill press using drill bits that just twisted into the main channel as mandrels to spin it. I could then shape the bolts with sandpaper down to a perfect fit in the body followed by shaping of the last 3/4" of the nose to seal perfectly into the back of the barrel. I plan on using a .684 insert most of the time, and will be going larger if needed, so the .684 was my sizing model. PETG smooths up well with the following steps .

120 Rough sizing
360 Finish sizing
600->1200->1500->2000->brown paper grocery bag->microfiber towel Polishing

I ditched the flat ramp to prevent ball chopping and went to a rounded nose. Seems to have the same effect, and its way easier to finish.

I also added a few millimeters of extra beef behind the pin. It has a 5/8" long 3/8-16 adjustment bolt in a 24mm long threaded channel, so it is always fully in threads, regardless of the adjustment position. The plastic seems to hold the velocity adjustment bolt firmly without need for the wacky ball in notch arrangement needed with aluminum threads in the original bolt. The Mk XII bolt has been working well, and I am confident on taking it out on the field.



I shot 500 or so balls trying these parts. Only had one ball break, but it was a strange one that showed a change was needed. On one shot, I felt paint mist shot into my glasses from the gap on the back of the feedneck. I opened the gun up and the ball in the chamber was fine. There was no paint in the breech or the barrel. It was the 2nd ball up the stack that had been broken in by the ball in the breech when the ball before fired. The pressure shot paint up into the hopper. Clearly, there was pressure coming around the bolt. I added the square channels that were cut under the original feedneck block into my design. Once again, the designers knew what they were doing. This, and subsequently better sealing bolts, made the issue go away. I can put two balls into the feedneck, and the top ball doesn't move a bit when firing.

[Calvin]

Now that it was running at field velocity with a good adjustment range over and under, it was time to start thinking about a field usable setup. I dug my most beat to shit AGD vert adapter out of a parts box and cut the ears off the top. Looking at mockups, this is what I really wanted to do.



Unfortunately, I don't have a mill to contour the vert adapter to the body or make a seal of some kind between the body. Gonna have to think about that.
Instead, I decided to mount it at the very front of the trigger frame. I drilled the mount hole there and cut off about 2mm of the mount block in the body under the screw for the vert adapter. It all came together well, and is very solid. I also have a nice cap that would remove the front silver ASA, but I couldn't find an acceptable hosing setup due to the short distance to the side of the body by the volume chamber. That will have to wait for the second body. Instead, I dug out an old Taso 1/8npt to ASA adapter and drilled in another 1/8npt hole to make the hose magic happen along with a gauge on front. I also whupped up a better cocking knob with a nylon tube over the bolt so it doesn't scar the body like the steel ones do.



So, the first of the two is done and field ready. What a fucking beast. I now remember why I loved using these things. The front hose bending upwards makes hand positions throughout that area comfortable. I should get out to the field next weekend to eliminate some folks 90s style.





I just ordered some double trigger shoes. Will get one on there ASAP. Also took a look at the travel for some forward and backwards stops. The forward motion looks to be almost entirely needed for the disconnector to disengage, but there is plenty of travel to be eliminated post seer release. Looks like a set screw into the grip mounting block is the best plan of action.

The grip that I have on there has really grown on me, so I will use my Impulse frame on the second body.

Some things I would like to do this winter:
1.) Find someone who can slab the sides and remove the back half of the top tube. There is a ton of useless weight there. Would send it all for some anno when done.
2.) Fab up a pull pin to hold the rear bolt plug instead of the two small and one massive screws. This would make disassembly and cleaning way easier.
3.) Find someone with a lathe who can lighten a couple bolts a bit and cut the larger channel in the bolt for a regular oring. (I'm seeing tons of old posts in various places that discuss the use of 111, 112, and 113 as the rings for this. None of them give precise measurements for the cut. Will have to research this more. Chaos, please share any details if you have them.)
4.)Find that same bottom line ASA in black. That blue is horrendous.

[Chaos]

For me I just kind of eyeballed it I didn't have any precise measurements when I did my original mod. I just chucked it in a drill press and while it was running I took a Dremel with a grinding bit and very carefully ground out the channel that hammer seal sits in until it looked like it would work. And then put I believe it was a 7/16 o-ring in there and it worked perfectly. Other people have taken this mod further and they're done cleaner versions of it so you might be able to get more precise measurements from them. If it helps you at all I have all the measurements of the stock hammer that was written out by a guy on the old VM Empire that I had gotten a titanium Hammer from. I also have all the measurements of the turbo valve should you wish to recreate it.

[Chaos]

I love the progress you're making on this. The internal mods that you're doing kind of reminded me of something one of the OG VM Empire came up with. This is the Assassin kit and it was designed to be a Homebrew version of The Old Pro-Comp kit. Not sure if any of this would be beneficial for you but you're kind of going along the same root of the looks of things and I definitely approve 🤘

[Calvin]

That pull pin for the rear plug is exactly what Im talking about. It is going to happen in my build. Just gotta survey my hardware options to make it happen. Breaking this thing down will be a breeze with that change. I will do some more research about the oring mod. My desire is to to a couple lightened bolts, and have the oring mod done at that time. I need to know what to ask for from such a person. I might see if Doc will do it for me. He did the mod in his VM video, so I know he has the specs. He's a busy man, so I hate to bother him. Till then, my printed PEBA ring is doing well. I would guess its at 1200 cycles without any signs of breakdown. PEBA 90A is pretty tough stuff.

[Calvin]

Thought about this over coffee. If I do get the body lightened so that the rear half of the top tube is removed, I can extend the back of the bolt out into a knob for cocking. With the cocking knob and rotating collar gone, you have a little more length of spring to work with behind the hammer. Things to look forward to.

[Calvin]

I worked up one of the stock valves I have to see if I could get the perfect output hole size. I did a subtle enlargement of the inlet port to 6.35mm, with my original modded valve having a 7mm inlet hole. My earlier guess was the proper outlet port would be 4.5mm. To be conservative, I started with a 4.2mm diameter hole. I didn't seal up the other ports at first because I also wanted to measure the effects of doing so. It turns out that 4.2mm is just right on my build, with my velocity adjustment bolt being dead center in the bolt's air passage at field velocity. I then drilled and tapped the other ports for 8-32 set screws and added 10-32 set screws to the two valve mount holes that I wasn't using. This gained me 10fps, so it does make a difference. A tad over 1/4 turn of the adjuster screw put me right back at field velocity.

The local field is doing glow ball on Saturday, so I will definitely be there from early afternoon till midnight. Lots of time to test this new build. I know my arms won't handle this beast the whole time, so some meched Ions will be my backups. I ordered an aluminum double finger trigger shoe from Ron-ASP, but I want the gun finished so I don't have to think about it before Saturday. I traced the trigger on paper and scanned it to png. Then I converted it to an stl using an online converter so I could import it into Netfabb as a model for a cutout hollow in a quick build of a double finger trigger shoe. I used a flush fit 3mm bolt to pinch it together onto the trigger and placed that assembly in the center of the curve in the grip mount block for dual use as a trigger stop. Took a little cutting and sanding to get it the exact depth plus a little seer release margin. Now its working great. It has rounded edges on the front, doubles the width of the metal trigger, and feels really good to use. I'm not sure if it doubles RoF from the factory single trigger, but its close. The gun is never going to be a RoF champ, but this will allow me to put down enough paint to get hits when the shots are difficult.



I also have left it aired up for two days. I wanted to see if the converted Tippmann valve stem would give me any issues. I am happy to report that it seals up tight as a drum with zero lost PSIs over that period. Inspection of the stem on removal showed a nice seal indentation. Looks like that conversion will work out well.

All is ready for the maiden voyage. Fingers crossed that it runs good.