Summary

There are a lot of instructions for making a spring clothespin into a toy "gun" for shooting matchsticks or toothpicks. They work OK, but need some fiddling to get them cocked after shooting--the spring and matchstick often wedge, or the spring refuses to go into its latching slot. Printing your own allows making some improvements, like adding a plunger with a stop and a shaped end that transfers the spring into the cocked position. And, it has sights...

Instructions

I use ABS in a FlashForge Creator.

I print the plunger using solid fill, standing on its flat end, no raft.
I print the barrel standing on its output end, no raft. Either solid or 5% fill seem to work, but I'd think solid would be more durable.
I print the grip using a raft and support, barrel side upward. Leave the raft in place until you've finished gluing.

I trim the plunger edges to make sure it's smooth, using a scraper or a wide-blade snap-off knife.
I scrape the slot in the barrel where the plunger has to slide, to make sure it's smooth.

Insert the plunger into the slot, with the flat end toward the front (exit end) of the barrel. Make sure it slides freely.

Insert the spring. A plier may be helpful. Insert one leg into the front crossways hole, then distort the spring and insert the other leg behind the plunger, so the plunger is trapped toward the front of the barrel. That keeps it out of your glue...

Press the barrel into place without the glue once for practice.

Put superglue (I use Gorilla) around the recessed edges on the upward-facing grip, including on the semicircular protruding post's curved surface.

Press the barrel into the glue and let the glue set for an hour or so.

(You can use acetone instead of superglue, but I find it harder to control.)

When it's strong, you can cock the gun by ramming a stick down the barrel. The spring should slide off the plunger into the cocked-position slot. This seems to work better the second or third time.

Insert your ammo (I've even shot bamboo skewers--it's pretty powerful). Matches with the heads removed, or single-ended toothpicks, work well too.

This, like its clothespin cousin, is powerful enough to be very dangerous to eyes, so be careful where you point it when it's loaded.

And now, you know how the first thing everyone says when you tell them you've got a 3D printer is "Are you going to print a gun?!!", now you can say "yes, want to see it?"

Toy clothespin-style matchstick flinger