Just1ChanceFree, Here's the data for a #330 O-ring in this application: CYLINDER DIA: 2.469 O-RING NUMBER 330 O-RING I.D.: 2.1 O-RING O.D.: 2.52 O-RING CROSS SECTION: 0.21 ID/OD INTERFERENCE: 1% INSTALLED O-RING CROSS SECTION: 0.208 EXTRUSION GAP: 0.015 (GAP BETWEEN PISTON & CYLINDER) GROOVE BASE DI...

I like the innovative design. Especially the trigger mounting system and the O-ring seal on the barrel.
Clearly you have considerable metal working experience under your belt.

Gippeto- You are correct, thanks for catching that.
My oversight.

Any welding supply store will have the correct nipple for using this regulator with CO2.

I've been thinking about a hybrid, but first I'm going to try going supersonic with 200 PSI helium... just to see if it can be done. I'll try the used marshmallow idea. Might be interesting! Also, I'm going to paint some marshmallows fluorescent orange (to make them easier to track) and see how far ...

Thank you.
I've been pouring over this forum for about a month now, and I'm pleased (impressed?) to see the very open exchange of ideas & feedback.

My god man!! I think you've hit on something here :D A laser sight would be perfect ! The problem I'm having is that I seem to be losing interest in this gun because I'm already fixating on the next (newer/bigger/faster/taller) gun. Is this the normal sequence of events in the life of a spudgunner? ...

Most productive tools: Paper, pencil, eraser, calculator, computer.
Tools used most in fabrication: Angle grinder, TIG welder, Drill press.

I did for the first one, but I have a machinist friend who turned this one for me.

Twizi: That is indeed a pvc barrel, pressure rated at 465 PSI. These shots were made with 160 PSI air. Ragnarok: Yes, I know that OSB is not technically plywood, but it was the only thing I had in the bone pile at the time. My goal is to punch through 3/4" true plywood. -More on that as events unfol...

I was thinking about my grandkids’ marshmallow shooter, and started wondering "If you accelerate a marshmallow to a high enough velocity, would it be able to penetrate plywood?" yep. Many thanks to Technician for his piston valve design concept & inspiration. [youtube]http://www.youtube.com/watch?v=...

Here are some specs I worked up for an O-ring & piston based on the Dichtomatic O-Ring Handbook: Pipe size: 2-1/2” (2.469” ID) O-Ring # 330 Piston OD: 2.439 O-Ring groove diameter: 2.121” O-Ring groove width: 0.285” This will meet all the criteria for ring squeeze, compression ratio, extrusion gap, ...