Sounds good. Thanks for all of the replies everyone.

I replaced my Longshot's plunger head with a hereticorp/CS style plunger head. The plunger head and plunger tube were both lubed with white lithium grease from Liquid Wrench. When I first fired it, ranges were 90-100 feet. But within five shots, the range had dropped ten or fifteen feet. When I opened it up, I found that the rubber washer seal was TOO good: It had pushed all of the lube down to the end of the plunger chamber. The lube had also bunched up around the rubber washer. I don't want to revert to an O-ring seal, but I'd prefer not to open the Longshot every five shots and cover the shaft in lube. Has anyone else ever encountered this problem?

I didn't think pictures were necessary by the way I described it, but if anyone needs to see them let me know.

P.S.- If it really matters, I'm running an AR-15/LS spring combo.
Thanks in advance.

If you want full-auto, hopper an RF20 piston. Easy as pie. Just use CPVC to PVC couplers, or a DMK wye.

That's a good idea. I'm going to try this, and post results later. Gracias!

I planned to have a large air resevoir tank made of PVC and endcaps, then attach it to the firing tank. You could have a semi-auto if you attached a barrel system straight to the firing tank, but ideally the valve I'm looking for would release the air when a certain pressure is reached behind it. This way, when the trigger is held down the air won't just blow through, it actually gives it a chance to pressurize. If you wanted both, you could set up some ball valve system.

Okay. So I'm looking for an air valve for a potential automatic homemade. Obviously I've googled it and I can't find what I'm looking for, so this is the next best thing. Anyways...
I need a valve that releases air when a desired pressure is reached. It's similar to a pressure release valve, but those wouldn't direct all of the airflow into a barrel or something similar. Any ideas?

Very nice. Do you think you could install barrels that extend to the front of the shell?

If you extended them all of the way to the front, then it wouldn't rotate.

I had this problem too. I used hot glue to secure my catch spring(s) to the catch piece. The glue created a kind of spacer underneath the springs, so the spring gets compressed more.
I know the standard size that is used for AR-15 springs is 4 3/4'', and the AR-15 spring and the [k26] aren't that different.
PM me if you get it working, I'm curious as to how this turns out.