This is a gun I made which was originally a grey connector speed reloader, but I modified it into a powerful gun. It has 6 round cap, but I might post a 23 round cap version sometime, if you can't work out how to (just extend, and replace the yellow push rod with a grey one). To fire push yellow rod up until it fires.
Pros:
Semi automatic
Powerful
Quick reloading
Low piece count
Cons:
No trigger (push rod to fire)

Step 1: Piece Count

Please correct me if I get it wrong.

Rods:
Green - 7
White - 12
Yellow - 1

Connectors:
Yellow - 8
Orange - 4
Dark grey (excluding ammo) - 4
Red - 4

Other:
Black Y-clips - 2
Elastic bands - 1 needed, 2 wide ones recommended, you can use just like 10 quite thin ones

Total (excluding bands and ammo):
42

Step 2: Magazine/grip

More or less the whole gun. I will help if necessary.
9. The white rod doesn't clip in to the Y-clip, it just rests there

Step 3: Other Parts

The firing pusher thing and the thing to stop round falling out. The instructions are simple enough I think.

Step 4: Loading+firing

1. Attach rubber bands.
2. Load the rounds. It doesn't matter which way you load them, but the way the are loading in the photo makes them fly straight. The other way makes them fly slightly up. It doesn't improve range, just if you may want to hit something behind e.g. a sofa.
3. Push the thing I'm pushing in the photo.

If it is not powerful enough add elastic bands until you get the desired power. The more bands, the harder it is to fire.