Hoppered HAMP Instructions
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.
If you plan on using standard Nerf darts, you can follow the simpler instructions for the singled HAMP, which is not semi-automatic, but can be used as an impressive Nerf shotgun.
This video shows a slightly different design with 3d printed brackets, but it is functionally identical to the version described here.
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 that are explained at the end of the instructions.
PreAssembly
1stTape
Crooked
Straight(er)
MoreStuffing
Stuffed+Glued
ExtraTape
SlidingStuffing
Resized to 88% (was 1632 x 1224) - Click image to enlarge
If you're going to use these for marshmallows, there are a few differences in design and operation.
-If they get too melty they will get unremovable gook all over your barrel that will prevent it from working again.
-If they get too cold, they will be much less soft, which leads to both more target pain and fewer projectiles fed per shot.
-Stale marshmallows act like cold marshmallows. They get stale fast.
-You CAN use an easy-to-find Tee fitting instead of the wye fitting.
-Use 1/2" Thinwall pipe for the magazine. It should have an ID of nearly 3/4", but still fit in 1/2" pvc fittings.
-If you can't get 1/2" thinwall pipe, try to find something with close to 3/4" ID, and just improvise at the coupler point. Probably lots of duct tape, and you will have to reload from the cap.
-Try sch40 1/2" PVC if you feel the barrel is too tight for your marshmallows. Sch80 is usually good for most mini-marshmallows.
-If the marshmallows aren't feeding enough, you can use a follower. For example, a 4" long stick 1/2" round of nylon cannot reasonably be forced through the wye, but will push on the marshmallows in the magazine if placed on top of them. This makes reloading more of a hassle, but will usually result in more reliable and impressive blasts of marshmallows. Just make absolutely sure that the follower can't be fired.
-The amount of marshmallows fired in such a short amount of time can make cleanup taxing, depending on the environment. Ideally find a place where they can all be swept up, or a place where they don't need to be removed from.