Using the barrel as the pump.

[Demon]

Just thought of this.

The under-barrel pump rod pivot near the pump's head. This engages the pump rod in the barrel to compress the air.

The air chamber and the barrel communicate with a one-way valve, allowing the air compressed by the pump to directly skip to the air chamber.

... And there you have a complete onboard system. The ammo is inserted with the first pump cycle.

Only concern is pumping when the valve misfires. But doing so while crouching, and with the weapon's axis directed elsewhere than any part of the body should be alright.

[Gun Freak]

Very interesting idea! I would load the ammo after pumping. Don't quite understand how the valve would work though. This seems like a cool idea but it could be very unsafe because of what you stated. Still a cool idea.

[Demon]

Loading the ammo on the first cycle avoids doing it after, when you are pumping.

Pumping with the pump's handle under the foot and weapon firmly in hands would be safe.

Valve's a simple inline one. Nothing fancy.

[Gun Freak]

Ok, yeah. But what about the valve? How would a one way valve turn into an exhaust valve?

[Demon]

?

The one way valve is meant to get the air from the barrel to the air chamber. It makes it skip the main valve.

[Mr.Tallahassee]

He's using a bypass valve to pump up the chamber through the barrel. The main exhaust valving would act when firing but not while pumping whereas the bypass valve would only act while pumping, not firing.

[motorfixer1]

This is a cool idea! The pump rod acts like a ramrod? For ammo and air? Also, what type of main valve would you be using? Looks nice and compact.

[jackssmirkingrevenge]

interesting idea.

have a look at clide's minipump: http://gbcannon.com/projects/launchers/minipump4.html

[Fnord]

Depending on what you're shooting, barrels tend to get a little scratched up inside. Other than that I think this is a brilliant idea, it just needs some more thought as to ergonomics and safety.

[Demon]

The ammo is encased in a foam sabot, so I don't think there's any issue on that side.