I apologize for the necro, but this adds significant content.
After building several of these bows for my friends, I found that they shattered easily at the point where the hole was drilled. There simply wasn't enough material to withstand the constant bending.
I have a solution. First, use a 1" x 1/2" tee. Then, bore out a 1/2" coupler, so PVC slides through it easily. Drill through this, and the bow arm. Slide this into the tee, and follow Ice Nine's instructions for the rest.
This seems far more durable than the original design, and I have put hundreds of shots through it with no sign of wear.
I want to make this clear: this is the first bow-powered +bow.
This was an idea I had and discussed it with CaliforniaPants. Since I lacked a 5/8" spade (basically necessary) at the time he built the first prototype, and said it worked great. Supes props to him, and he deserves a lot of the credit as I probably would have sat on this for a while longer unless he hadn't tried it out first. I finally picked up the parts for this, including the bit (total cost: $12), so here's the write-up.
This tee is 1/2" input with 3/4" sides. Pretty simple stuff, throw a bit of 1/2" in the input.
Sand out two 3/4" to 1/2" so 1/2" PVC can slide through them without too much force.
Slide four feet (or whatever) of 1/2" PVC through the bushings.
Take your 5/8" spade bit through the input. I did it with a drill, not a drill press, and it was a bit tough but totes doable.
Feed 1/2" CPVC through the whole setup. Cut it down at the end and put a stub of PVC on it for coupling purposes.
Solvent weld the PVC over the CPVC so it doesn't leak and doesn't get pulled out.
Then I pulled the spring out of my +bow.
String 'em up. Variable tension is pretty nice and it's very easy to get the same amount of power out of this gun as with a K26.
Throwing a hopper on it is the ideal.
Doing this to a +bow may seem odd to some people, since it's a lot more work and a decent amount of "larger gun" as compared to a K26 +bow. However, as people with bows and pump-action crossbows will tell you, bow arms are much much smoother to use as compared to the spring. While supplying the same energy, the bow arms are a lot easier to prime and a lot smoother too. I'm definitely a fan of the spring never rotating so as to grab on the catch notch. I would prefer to use a bow on this for the most part.
The best part of this is that the setup is entirely modular. It's very easy to untie the arms and take it off the gun, and as most +bow owners know it's simple to put a spring in the gun. You could then use the same arms on SNAPs, +bows, a whole host of guns, really anything that has an external priming arm (or even internal, if you were willing to cut the shell up).
Edited by Ice Nine, 29 May 2015 - 07:17 PM.