That's right, a repeating crossbow.
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Step 1: Barrel
1 - Rails. Make two
2 - Barrel walls. Make two
3 - Barrel floor
4 - Add the floor parts to one of the walls with white rods as shown
5 - Add the other wall
6 - Add the rails onto the sides of the barrel
7 - Closeup of the back, showing how to secure the rails
Step 2: Trigger
1 - Make
2 - Make
3 - Make
4 - Put them all together
5 - Add to barrel. Shown without the rails for clarity
Step 3: Handle
1 - Make
2 - Put the pieces together with three white rods
3 - Attach to the barrel. Shown without rails
Step 4: Magazine
Or just the top of the crossbow in general
1 - Oops, sorta blurry. Walls of the magazine
2 - Sides of the magazine
3 - Attach four white rods at appropriate places on one wall
4 - Slide side walls onto previously mentioned white rods
5 - Attach the other wall
6 - This goes on the back for a little stability
7 - Attach the magazine to the barrel
Step 5: Bow
What's a crossbow without a bow?
1 - Two stiff, gray-sized rods in the front of the barrel. Connect at the ends with light grays
2 - Add two red connectors and two white rods to each side like so
Step 6: Pump
1 - Make
2 - Make
3 - Add two yellow connectors to what you made from pic 2
4 - Connect the two things together
5 - Add the white rods and yellow connectors to the ends
6 - Prongs - they hold the rubber band. Make two
7 - How the pump attaches to the barrel. It takes a little pushing and pulling to get it around the rails
8 - Attaching the prongs to the pump. The white rods just slip into the hole of the middle yellow connector
Step 7: Finishing
1 - Three bolts is all it can hold
2 - Pusher
3 - Two #64 bands tied together function as the bowstring
4 - Attach the tied bands around the front white connectors
5 - Rubber band for the trigger. Hook it in the orange connector and then around the white rod. This makes it not get stuck when you pull it down
6 - Bands for the pusher
7 - How the bolts should look inside the barrel.
When the tied bands are pulled back, they should go over the angled edge of the trigger and hook on the back. With those bands out of the way, a bolt gets pushed down into the barrel (note - sometimes the magazine and the barrel aren't exactly aligned. If this happens, just push the side of the mag a little so they line up). After firing, the system will reset. Just repeat.
Happy shooting and comments/criticism always welcome!