This is how I built my single-action barrel sealing piston valve air gun. This gun shoots as fast as you can pull the trigger and really exceeds the power and velocity that one would reasonably want to shoot marshmallows at. Please don't build one with the idea of using it for marshmallow war games. I know from personal experience that, getting shot with a nice, fluffy marshmallow out of this gun will raise a serious welt, even through denim. Who knew that something so soft and fluffy could hurt so bad?
I believe the design is unique and achieves a faster rapid-fire rate than any other piston valve gun design, because it eliminates the valve that other designs require to isolate the air compressor before firing. This gun is pressurized through the pressure chamber, not from behind the piston.
This instructable is extreme, because it is the world's most powerful marshmallow gun - blowing away any other marshmallow gun to an extreme.