I modified a pistol that i found on this site.I based my design on twisted's pistol.

https://www.instructables.com/id/K3-knex-pistol.-*new*-shooting-system/

some of the good things about my design are:

easy to take apart
butt of gun is comfortable
somewhat revolutionary

There are of course a few problems:

weak and cannot handle more than 2 rubberbands
rapid fire makes some parts come apart
not always reliable

Currently i'm trying to find a way to put a magazine on it, but i can't find an effective way of doing so.

if you guys want, i can make a step by step instruction guide. But i will only do if a decent amount of people are actually interested in making this.

UPDATE!!!!!

i got the autoloader working, but it jams when you get low on ammo (about last 5 rounds)
It holds a maximum of 16 rounds
it still only shoots about 5 feet

Step 1: Basic View of Gun

this just shows a view of the gun as a whole. as you can see, it isn't all that long. So it's more like an SMG than a rifle.

Step 2: Slide Pulled Back

In this picture, you can see what the gun looks like when the slide is pulled. The firing pin is not connected to the slide in any way. Out of the picture, i'm holding it back. The slide just cocks the gun and gets the firing pin in the right position.