Hoppered HAMPs are manually driven, semi-automatic blasters that can be easily built with minimal tools for under $20. The power is partly a function of how hard you pull the plunger, so you can only use as much power as you need. As described here, a casual stroke will fire a 1 gram dart ~150 fps and a very hard stroke ~200 fps, but longer or shorter barrels can be used to increase or decrease the power of these sort of blasters.
This is a replacement for this old writeup with dead pictures.
Many of the instructions here are the same as for the singled HAMP, but for clarity they are repeated here so that a complete guide is presented for these blasters.
These instructions mostly explain how to make these for special hopper-friendly nerf-like ammo that is difficult for many people to make or obtain. However they can be used to produce a semi-automatic marshmallow shotgun that uses extremely cheap and ubiquitous mini-marshmallows. There are some additional things you should know if you are going to do this. See the second post.
Edited by Aeromech, 23 November 2015 - 01:08 AM.