Twitchtech 2-pump Chump
Posted 30 January 2008 - 11:10 PM
Pump from a 2nd Airt3k
Great Stuff Insulating Foam
Tube connectors for common fish tank tubing
Here's my creation:
The idea is that by reversing the plungers, to prime the tank, all you have to do is press into the shoulder stock with your shoulder. This would also allow the stroke length of the plunger to be nearly doubled, and with the addition of the 2nd plunger, each pump puts roughly 4 times as much air into the tank. (1-pump At3k mod.) The reason I did this rather than the more common 1-pump modification is because with this set-up, the auto-rotation mechanism still works. Thereby increasing RoF significantly. (1 pump for greater range, no need to manually rotate the turret)
And how to get what I managed thus far.
cut the pump from a 2nd Airtech assembly (preferably a broken one), and cut away the goo gauge from your intact one. just snip the line at each end with scissors. From here, find a T-Junction piece from the package of tubing connectors. I happened to use connectors I found at a pet sore, used for simple aquariums, to rig an air line to the tank filter, those goofy bubble-emitting decorations, et cetera. (I started this project before I knew what instant connectors/barbed connectors were.)
You'll have to cut off the back of the At3k, behind the trigger, and a groove for the reversed plunger.
What's with the metal rod? It's attached to the shoulder stock, so when the stock/plungers are compressed, to tooth glides forward and back (the spring pushing the plungers back out of the cylinder) activating the rotation mechanism.
You'll need to do a bit of cutting so the upper plunger fits as well.
Because I wanted to keep the tubing from interfering with the motion of the advancing tooth, I connected all tubing outside the shell of the gun. (which actually worked fine aesthetically, since the At3k has the fake orange tube embedded in the shell anyway.)
To do this, I had to make a bit of a hole, so the tubing could slip outside the shell.
Next, the creation of the shoulder stock. a 5/16-18 threaded bolt fits perfectly into the hole in the plunger rod. The Home Depot near my house carries it in lengths as short as 12", but you only need about 6"-7."
(That's what she said?) I used my cutting torch (MAPP gas) to sever the excess. I imagine the rest of you can use other methods, though certainly none as fun. Next I grabbed a few wing nuts and washers and had the spacing between the two plungers set. (you obviously want these parallel.)
This spring pushes the stock away from the gun, thereby cocking the gun (the mechanism advances the turret on the backstroke)
To make the stock more comfortable, I coated the bolt & nuts with Great Stuff insulating foam, then trimmed to fit once it cured.
(has not been trimmed in this picture)
The length of the rod needs to be cut (again, I chose fire.) such that the tooth just nearly bottoms out against the rear of the tank, when the plungers bottom out in the cylinder.
In the picture, you can see a hole in the base of the trigger. I placed this as a guide for the rod holding the tooth, so it wouldn't wobble left/right. In the picture, only 1 rod is present, but after testing, I recommend a 2nd rod as well, to keep the tooth/rod from being twisted about the axis of the rod as it is pushed past the advancing mechanism.
Without the pump handle being located in its normal locale, there now exist a nice spot in the front of the gun, below the barrels. facilitating another integration of your choosing.
At this point, you can close it up, and have a functioning At3k that gets more pressure/pump, and the auto-rotating mechanism still functions. Here's why I still consider the modification a failure.
As stated in the beginning, ideally the stroke length of the plunger is nearly doubled, resulting is nearly four times as much air being pushed into the tank. As it rest on my dresser right now, this is not the case (only twice the volume of air.). Because the plunger's range of movement is being doubled, the advancing tooth's range of motion must be doubled. However, the trigger gets in the way, preventing the tooth from sliding back any further than it originally could. I see 2 options, and seek counsel as to how to best rectify the problem.
1: Find some way to let the plunger/shoulder stock, etc. more twice their stock distances, but the tooth it's normal distance.
2: Relocate the trigger.
I'm thinking the first one would be best solved via. leaving dead space between the stock and rear of the tooth's guide rod, with a compression spring pushing off the back of the gun, and pulling the tooth back to rotate the turret. The 2nd method I think could be best solves using a wire and pulley to relocate the trigger, but still keep my hands in the same place.
Thanks for your time.
Posted 30 January 2008 - 11:37 PM
I also happen to have spare AT3k's so I will be attempting to create something like this soon.
Edited by Peter, 30 January 2008 - 11:38 PM.
Posted 30 January 2008 - 11:37 PM
First, put the advancing tooth on some sort of track. Actually, if you would have cut off the entire white end of a pump shaft, you could have just used the rail system that already existed to guide the pump when stock.
Then, you make the metal rod able to slide forward through the advancing tooth. The rod should, hopefully, be able to move forward past the air chamber as the stock/pump is pressed inward.
As the pumps are drawn back, the rod is pulled back with it. At the correct time, the rod will begin to pull back on the advancing tooth.
...and that's about it. The only problem might be that the rod will eventually run into the turret. Still, getting past the air chamber will at least (if memory serves) give you an extra inch to two inches of pump travel.
Awesome mod (and name), by the way.
Edited by jwasko, 30 January 2008 - 11:38 PM.
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Posted 30 January 2008 - 11:43 PM
Second of all I think I may have a different approach to your rotating mech idea.
Is it possible to run some thin metal fishing line from the stock through the back of the gun and set up some small loops for the string to run through so it ends up behind the rotating mech that needs to get pushed back toward the shell?
What I am suggesting is getting rid of the rod all together. The flap that the rod pushes back is what rotates the turret. Attach a metal fishing line to the back of it and run the other end to the stock.
This way when the stock/pump moves backwards it rotates the turret. Then after you pull the trigger the flap returns to its original position then all you have to do is push forwards on the stock, this will allow the rotating mech to re-position itself, then the stock will be pushed backwards via the spring, the spring will also turn the rotating mech switching to a new barrel.
Getting the wire set up perfect is tricky but I can see this working.
I can just see this in action.
Thrust stock against chest.
Thrust stock against chest.
Thrust stock against chest.
I hope this works out for you.
I don't feel the way I used to do.
I know its bad,
After what we had,
But Iím just not the angel you knew.
Posted 31 January 2008 - 12:06 AM
How well does the spring return the stock? I had this image in my mind of it being pumped, and then slowly creeping back out. But, you can obviously tell me better than my imagination can, so I'll rely on you.
The turret rotating idea is quite cool, almost like your B20.
Posted 31 January 2008 - 09:13 AM
The spring I used is actually the stock spring from my Longshot (the Longshot has a BBB spring.) and can return it well enough. Because of the tooth being pushed past the mechanism it's not quite as fast as I'd like, but I have an extra AR-15 spring laying around and will probably put that in there instead.
Edited by Twitch, 31 January 2008 - 09:13 AM.
Posted 31 January 2008 - 09:48 AM
Posted 31 January 2008 - 01:02 PM
This is more of an observation than a suggestion...but I was thinking that the auto-rotation won't make thr ROF any faster if you're not pumping with your left hand. The reason some turrets take longer to reload than auto-rotating turrets is that the cocking/priming action is done by the same hand that rotates the turret. In this case, however, your left hand is free while you're pumping so you can rotate the turret at the same time that you are pumping the gun. I know that this doesn't solve your problem, but rather argues that there is no problem. If you use your left thumb to turn the turret while you're pumping you can lengthen the pump stroke and fire just as quickly as if the turret rotated itself.
My concern about rotating the turret manually is that I personally would rotate the turret a click too far, or too short, and the gun would misfire. I suspect the overwhelming majority of those who read this would be able to do just fine, but I am paranoid beyond reason.
Another Point I feel I should make, The original intention was that with the turret rotating automatically, and the gun only required 1 hand to prime, that I could in theory dual-wield a pair of these. (Hence my desire to ensure the rotation mechanism was operational)
I'm thinking Forsaken Angel probably could, but I can not. Despite all my knowledge of chemistry and physics (I'm applying to medical school at the moment) I forgot that it would take a much greater force to compress 4 times the volume of air in a single pump, instead of 4 smaller pumps. The end result is that the gun will most certainly be wielded with both hands.
With that in mind, yeah, manually rotating the turret wold work just fine, but there's always room for improvement. And I'm paranoid/neurotic.
Posted 31 January 2008 - 02:51 PM
I'm joking. I really like the idea of it. Great stuff won't hold though. Is the spring powerful enough to push both of the pumps back?
Posted 31 January 2008 - 03:47 PM
sn1per I appreciate your humor, that made me laugh literally out loud.
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