I built this one not long after the first v4.1. The design is the same as v4.1 in my previous thread, but scaled down a bit for a more compact and less powerful design. It is 3" shorter (4.25" long) and shoots around 100 fps. The main thing I like about this version is it can be pumped between your hands. The previous version could not(not by me anyway). Its quite comfortable to hold because the sleeve fits perfectly with the width of my hand. Anyway, here are the pics.
<a href="http://gbcannon.com/pics/minipump/v4.1/mini/size.jpg">Size comparison to a couple things while loaded</a>
Its been holding pressure wonderfully. I've left it charged for over a day several times and it still fires very strong every time.
Other than the occational botched pump, due mostly to slightly too much rubber being taken off the piston o-ring, I haven't had any troubles with it.
previous thread explains how it works and has pics of the inside...Looks the same on this version.