The Evil Twin

[DocsMachine]

Okay, from the initial parts-box build, we had several issues.

One, the valve would stick open making it hard to first pressure up; two, the timing was not cooperating and I kept having 3-way issues; and three, there was a wild variation in velocity between the barrels. Like the top one would be near-normal, while the other would be "walk two steps over and pick the ball back up".

So, this morning, I broke it back down and took each one in turn.



The valve issue was my own ignorance. I'm using an off-the-shelf Inception 11/16" valve, and had machined the valve bore with a 'step' for the valve to seat against. Some markers don't do this (like Trilogys) while others (various Sypder clones) do. I wasn't sure, so just to be on the safe side, and to make it easier to assemble, I had a slight step there.

And Inception valves have "wings" on the cup seal, that actually very nearly touch the valve bore- and my bore was smaller than it was supposed to be- not by much, but enough the cup seal dragged. That was an easy fix- rather than bore out the body, just as a quick fix I trimmed down those "wings" by about 20 thou, and now it works great.

The timing issue was simple. The first 3-way I'd used was an older Quickswitch, that, frankly, has a lot of miles on it. I've had it on at least two dozen builds over the years.

I just threw a new set of spool O-rings at it, and it worked like a champ. The rest of the timing was easy, once the 3-way cooperated.



The velocity issue wasn't much harder. Clearly what was happening was the air burst sailed right through the lower bolt into the upper. So we needed a little restriction in the upper, to kind of force some of that air out the lower bolt. I got out some more Delrin, and whipped up a very rough prototype with an internal adjuster.



Which is simply a long flathead screw, trimmed down slightly, and with a 'step' machined into the bore of the bolt to the point where the screw head can nearly close off the airflow completely.

Ideally, a bolt with a fixed, reduced bore in the upper would be preferable, and I'll try one of those next. But for now, this got me running. A couple quick test shots and a slight tweak, got the barrels very nearly identical, and a little tweak to the velocity got up into the 260s. (Still running straight CO2, maxed out at around 600 psi. )

Finally, it was time for a proper test. I dug out a pair of old VL2000s, as I was too low on fresh 9-volts to run a pair of Revvies...



And dug out some old test paint to give 'er a try. I had half a case of Valken left from when the local field closed a couple years ago, and this was the highest and best use.

Or maybe not.




The stuff was one solid lump, even though it had been stored warm and indoors since I bought it at the field, and the bag was still completely sealed.

Just how crap IS paint getting these days?

Anyway, I had some other old paint to toss, that was in a lot better shape, and set up a video camera out my back door. For those that haven't seen it, years and years ago, I rebuilt the back door to the machine room, into a Dutch door, so I could open the top half, and had a "shelf" on top of the bottom half, to lay tools, the chrony (hanging to the right there) paint, loaders, Dew, Moose Repellent or whatever.



I actually have some hanging targets back there, but with all the junk in the way- like that vertical mill - I couldn't get a good angle on it.

But, without further ado, albeit with a bit of a preamble, the real, live shooting video!

As always, more to come! I need to fill my HPA tanks, set up a good inline reg and a nice rail ASA, make and test a couple more bolts...! Stand by!

Doc.

[BrickHaus]

My God thats beautiful.

[Chuck E Ducky]

It’s Alive! That’s awesome Doc!

[DocsMachine]

I've been running this thing essentially 'tethered' to a 20-pound CO2 tank. I've had this setup for years, as it's longer lasting and more controllable when testing guns.

But naturally, I want to run it as intended with a screw-in. I have the tanks, I have the compressor to fill them. I also have a boxful of secondary regulators. (Inline regs, "foregrip" regs, HPRs, etc.)

I have a ton of "low pressure" regs, but only a handful of "high pressure", and I had to rebuild four of them before I finally got one to work. (Lacking certain specialty seals, there's a limit to what I can "rebuild".)

And, I'll need to re-test even that one after I get full-pressure HPA again, to be sure that it, too, doesn't "creep".

Quick Q for MCB-at-large: I have yet to try an Inception HPR, anybody know what kind of pressure range they're good for?

Doc.

[Chuck E Ducky]

Quick Q for MCB-at-large: I have yet to try an Inception HPR, anybody know what kind of pressure range they're good for?

Doc.

75psi -Tank pressure (wide open) according to there website. I have only messed with the Empire sniper version and a few inception cockers but never had to pull one apart
yet. Really just sweet spotted them during tuning. It’s a nice reg.

I’m thinking of going with an AKA 2L+ for my V Twin with the HP Spring kit. Reg dyno shows them to have superior recharge rates. Something that might be important especially running 2 barrels off one valve. It’s probably going to need a fairly stiff spring combo to.

[DocsMachine]

I’m thinking of going with an AKA 2L+ for my V Twin with the HP Spring kit. Reg dyno shows them to have superior recharge rates. Something that might be important especially running 2 barrels off one valve. It’s probably going to need a fairly stiff spring combo to.

-In the video, the gun was shooting around 260, at about 600 PSI of CO2. The hammer spring is kind of heavy but not absurdly so, although I did go through my stashes and found the heaviest hammer I have. (An old brass piece that I think was a Shocktech. According to my postage scale, which is probably not the most accurate thing, is weighs 45 grams.)

I suspect the Vee-Twin will be somewhat more efficient due to the better valve porting arrangement, but I'd imagine either gun will wind up needing 600 to 700 psi. And trying to go much lower than that will simply require heavier springs and thus a longer and longer dwell time, meaning less and less gas efficiency. (And this thing ain't all that efficient tot start with. )

Doc.

[DocsMachine]

You gents might get a kick out of this: I'd made up a rough test-bolt to try and regulate the air between the barrels. My plan after that was to make an upper bolt with a smaller bore, so there's no loose parts floating around. I just made one, with a starting bore size of 3/16"- .1875" (nominally, hey, I'm drilling Delrin. )

That's tiny. Most of you, if you'd gotten a 'Cocker with a bolt that small, would throw it away and replace it with the most cavernous one you could find.

Yesterday, in that video, with the rough adjustable bolt, I was getting around 260 FPS- the radar chron can't, of course, differentiate between the two balls in flight, and it'd occasionally "error out", but it was in that ballpark. I didn't go for long distance, but just by eye, the variation between barrels (with a loose-ish bore and literally six or seven year old paint) was probably 10-20 FPS.

Today, with the tiny 3/16" bolt in the top and the standard one in the bottom... I got roughly the same 260 out of the lower barrel, and (judging by eye) very close to 300 with the upper barrel. Yesterday there wasn't much difference between the two- today, there was a LOT.

I got MORE speed with the smaller bore. With an absurdly small bore. And interestingly, without any apparent loss from the other barrel.

Research continues, it seems.

Doc.

[Meleager7]

There is some funky evil voodoo happening inside Evil Twin.....

[Mr. Hick]

Sounds very similar on how they neck down HVAC duct work at the end of the run to increase air speed and deliver similar volume throughout the plenum.

[Alexndl]

Agree with Mr. Hick, turbulent flow should give you better back pressure than a somewhat laminar flow in a reduce section. The blocker in the flow will reduce the air speed with the turbulence created. In comparison the reduce cross section increase the speed. The bolt length is mode than 5 x the "elbow" radius so the flow should be laminar in the bolt.
But I could be wrong too...

[DocsMachine]

I first ran into something like this years and years ago. Mid-2000s or so. Customer sent me a P-blocked 'Cocker, maybe Shocktech or P&P or something, Can't recall. It had an Evolution-style "venturi" bolt, and one of the many mods he wanted, was a "better, high-flow bolt."

So I made one.



Thing was, the 'venturi' style gave an easy forty more feet per second. Put the venturi in, 285-290, steady as a rock. Make no changes whatsoever, just swap to the open bolt: 245-250, regular as clockwork. Swap back to the venturi, 285. Swap back to the open, 250. Every time.

One big trick to a 'Cocker bolt (and others) is to minimize how much the air burst can expand, between the valve and the face of the ball. Compressing the air is like pushing down on a spring- that's what gives it it's energy. If you let the burst of air expand- like into a big, open, cavernous bolt- that energy is wasted before it can do anything. You "let up" on that spring a little before it did any work.

But Orr told me, years ago- yes, I actually spoke to him personally once, there used to be pictures of it online to prove it - that he used to make the 'Cockers with the same size hole all the way from the valve to the face of the bolt. That gave the best gas efficiency with the basic setup. But the buyers complained about that- "bigger is better", and "bigger equals more airflow!" and all that. So the guns started coming out of the box with open, "high flow" bolts- the guns were actually somewhat worse, but the customers were happy!

Back in the Fastback days, I did the same thing- I made the bolts so the passages were the same size all the way through. And probably half the buyers ripped that bolt off and threw on some ill-fitting hunk of crap off an Impulse or Bushmaster or something, because it was "higher flow". 🙄

[/rant]

Anyway, I'm going to have to play with this some more, and I can see I'm probably going to have to waste a LOT of Delrin making test bolts. (I should make one with a removable head, like the old Hydras, which'd save material and time.)

Doc.

[Lt. head-shot]

Since this will likely be a similar issue on the Vee Twin (ie achieving high fps on a double barrel), Is there a chance venturi bolts could be made for those?

[Adam R]

Watching this come along is amazing. Thank you for the updates. I love both this and the V twin.

[DocsMachine]

Since this will likely be a similar issue on the Vee Twin (ie achieving high fps on a double barrel), Is there a chance venturi bolts could be made for those?

-No, there are already several 'Twins in the wild, and we haven't had the same issue. The air passages in that gun are much more uniform- there's two identical passages off the valve, one for each barrel, so it's a fairly uniform blast from both.

In this case, we have to route the air through one into another, and manage to get a reasonably uniform amount from each one.

I think Alex has it on the head- I need a baffle, not a choke. I'm thinking my next trial will be a pair of six-hole venturis, like the above P-block unit. Add enough 'drag' to both barrels, and maybe the air will have a few extra milliseconds to equalize.

And, I started wondering- how would that tiny-bore bolt work on a regular 'Cocker? Has anyone ever tried that? I've personally never seen one smaller than the diameter of the valve port, and even those are rare- most are "open as you can get without making the walls paper thin". I'll have to dig out a couple of my other 'Cockers and give it a try... when I have time.

Doc.

[Lt. head-shot]

The bolt on the KPCS Scout I had was only as wide as the valve port. I believe the goal was to try and boost the efficiency since its a cocker on 12 grams. As you've stated, less room for the gas to expand before pushing the ball.

[Trbo323]

Ghille-Man

[zinger565]

The whole bolt passage discussion makes a lot of sense, and is starting to tickle my engineering bug. I'm already planning on 3D printing some B2K bolts, so will definitely be experimenting with this.

I wonder if this would also have any application in the spoolie world, although it would be a bit more difficult to implement.

[Seajay]

The whole bolt passage discussion makes a lot of sense, and is starting to tickle my engineering bug. I'm already planning on 3D printing some B2K bolts, so will definitely be experimenting with this.

I wonder if this would also have any application in the spoolie world, although it would be a bit more difficult to implement.

I think that would depend on length of the bolt. A lot of spoolie bolts are pretty short so the length of flow is probably affected less. It might still "work" but not to any measurable degree.
I imagine it has a lot to do with the time the bolt lingers in the barrel area when firing, affecting air travel. That's my guess.

[cfos00]

I bore my bolts out to 3/8ths which isn't tiny but is still smaller than a lot of bolts internally, with very good results. I could make a smaller opening and see what happens on one of mine. That would push it about as much as you can with a cocker in terms of extreme conditions.

[DocsMachine]

Technically, even 3/8" is too big. The exhaust port through the body is based on a 5/16"-24 setscrew, used to retain the valve and of course plug the hole. The tap-drill size for that is .272", which is what WGP used to bore their bolts for.

.375" is 0.110 square inches of area, while .272" is .058 square- nominally half the area. That area of course affects the volume as well, and the idea is to try not to let the gas expand any more than necessary before it hits the ball.

Now, I'm sure it works- and in a typical 'Cocker, the difference is not an issue unless you're in a tourney and need to maximize possible shots-per-fill.

But in this case, the 'Twins (both styles) of course use roughly twice as much air per shot, with the Evil likely being a touch less efficient than the Vees. So that small difference, I believe, is worth chasing.

On the Evils, one thing I want to try, maybe with this prototype body, is a "choke screw" in between the tubes. Something simple like an older Tippmann velocity adjuster, and just adds enough baffle to the airflow that some is properly diverted to the bottom barrel. And, of course, adjustable, since we can't know if "matched" bolts, that work in my shop at my temperatures and pressures, will work on your field, with your setup.

I'm looking forward to getting back to this and trying it.

Doc.

[flyweightnate]

Try opening the bolt at a 15 degree angle - not sure how, without expensive tooling or some tricky compound work - like the tail end of a venturi meter. Air doesn't like sudden changes; anything more abrupt than 15 degrees tends to create eddies and shear. Running a cone-shaped hole might both allow the air to hit the whole ball evenly, but also reduce shear losses.

It also might not. Who knows. The expansion will likely be transsonic, which gets complicated. I'm not good at transsonic stuff, but done right, the wave front could hit that ball at over 800mph.

[zinger565]

Totally doable with a 3D printed bolt though.