Get a rubber band and a piece of knex chain.BEAT THAT EVERYONE ELSE!
If you want a realy strong gun which uses few pieces, I'd suggest making a slingshot type of gun. The body doesn't have to look nice, but you can make it long with few pieces which would make it strong. There's quite a few slingshot guns out there including ones by Masterdude (me) and Viccie.B1993 (Sipriani Rifle, quite good I've heard). All you should do is copy the trigger system and make the rest based on what pieces you've got left over.
This is very powerful but is a block trigger, but it doesn't use many pieces:<br /><a href="https://www.instructables.com/id/K_NEX-GUN/">www.instructables.com/id/K_NEX-GUN/</a>
Killerk's pistol is a good one. The ORIGINAL one, not the copies.
Actually I think the original killerk gun with a Magnum handle, so when you have loads of bands the handle doesn't come off when you cock the gun.
IAC Striker pistol is awesome, takes very little pieces, looks cool, and gets great range and power.
Here are some small but very good guns.https://www.instructables.com/id/Pocket-sized-Knex-gun/https://www.instructables.com/id/NEW-%26-Improved-k_nex-mini-bow/