Umarex MP5K

It's a BB gun I bought impulsively and played with once and has been stored in my closet since. The seals should be fine, considering I've never stored it with a cartridge inside, and it's been in my climate-controlled room/closet.

It's a knock-open valve system, which means a heavy trigger pull. That also means it's semi-auto only.

It's practically 95% plastic, with the internals and charging handle being metal. 

The blow back is just for the ejection port.