I bought some paint, I'm gonna go paint my guns on a weekend when it's hotter. I wonder if glossy paint would look nice...

it's not spring but air, the pressure of the air pushes the nerf bullet out.
That's why when people mod it, then replace the tube with one that leaks no air so much more powerful.

The worst nerf guns out now are the Recons and the Mavericks in terms of power and potential for modding, a lot of people would only get these guns for fun. Mavericks have too much leaks due to it's designs and any modding done to them usually loses their power.

http://www.buzzbeetoys.com/USglobal/RSAirBlasters/04100.html
this is one of the best guns you can get, with proper modding you can get up to 100 feet with it. Also it's one of the very few guns that can knock you out if you get shot by a fully modded one.
Air blasters, you need to pump the gun for each shot, that's why it's much more powerful but have a slower rate of fire because it's not like nerf where you just reload the spring. I want to get the rapid fire and shotgun because I hear the engineering copied the WWII guns 100% for the design except it just shoots nerf bullets.

That's called a O-ring that is used to give a seal so that air doesn't escape. Try going to hardware stores or car stores to find one that fits the size.