build something similar to a tennis ball thrower , and connect it to a big magazine
Well there are also turrets which are basically like giant revolver chambers. I recall we had one for 36 rounds. You could probably expand it even more. They are rather large but they're also a lot more practical than making say a 36 round box magazine. You could also make a chain. It's basically the same as a turret except instead of having the "rounds" wrapped around wheels or gears you have them in a long string. I actually wanted to see this idea because it would be you could completely select how large of a chain you wanted. You could try coming up with a dual stacking magazine. I tried it but it was rather difficult. Basically you try getting two columns of ammo to feed into one before entering the gun. It'll give you just under 2x the ammo though if you figure it out. If you're looking for an automatic gun then look at the answers below. I couldn't quite tell what you meant so I figured you just wanted a hi cap gun.
you'll need a semi auto or a machine gun, here's two examples:www.instructables.com/id/Knex-Machine-Gun_1/www.instructables.com/id/Truly-semi-automatic-knex-gun/
You'd need a chain fed machine gun. These are basically a few dozen block triggers on knex chain.