Things finally clicked for me when I saw Stark's fantastic RainBow. Needless to say, this catch wouldn't have happened without the Rainbow and the Minnesota crew. Major props, guys.
The goals with a CPT improvement was to get rid of the issues with nail length tuning and spring size issues, while making sure it could still be built with basic hand tools. Check and check. Here's how it currently looks.
The trigger installed:
Main deviation from the Rainbow catch is that it uses the clothespin to push up and keep the catch caught (whereas in the Rainbow, the push up fires the blaster). This keeps the motion the same as the old CPT. (EDIT: Yeah, the clothespin is backwards. I used a short piece of PVC for the test install, so it wouldn't fit the other way. The clothespin can go in either direction.)
Held in place by one screw and the firing pin:
The view of the inner workings, with the rear endcap removed:
The aluminum tube is screwed into the clothespin. The actual catch is made of 1/2" PVC with a hole to accept the aluminum tube. Currently, it's just pressed together, and it seems to hold fine. Later refinements will probably used threaded spacers from the hardware store.
The rear endcap keeps the plunger rod centered in endcap holes, and keeps the plunger rod catch from hooking on the front endcap hole. The front hole needs to be a bit looser than the back, to make sure that the plunger rod notch doesn't hook on the front hole.
Here's how the plunger rod is set up:
1/2" Aluminum tube nests in 1/2" CPVC with some convincing. Once again, a stronger spring may need a retaining pin, but currently, it's not going anywhere. I beveled the front edge to allow the plunger to keep moving if I overdraw. Otherwise, the plunger draw would stop suddenly when the draw reached the front of the catch indent.
Another view of the catch, with no plunger rod in place:
This is mainly a proof of concept, built with scraps around my workshop. Improvements can be made in the form of ease of assembly. Mainly, I'd like to see the aluminum tube be threaded into the catch, as opposed to just pressed in place. Still, even with this assembly, it built just fine, and required none of the tuning of a normal SNAP catch. The only slightly critical part is placement of the hole for the catch rod: it needs to be placed so the catchface is sliding up and down against the back of the endcap, so the force of the spring can be transferred to the front endcap. Otherwise, the catchpin will tilt, and cause similar issues to a standard CPT where the hole is too big. Then again, this system is probably a bit more tolerant of issues like that, since the catchface is in constant contact with the plunger rod. More building is needed to find out.
Edited by Carbon, 23 February 2015 - 11:32 AM.