I'm not sure how I'm meant to collect anything when attacking the enemies just sends them back to their initial positions. There must be an area I haven't found yet. Seems you just wait for them to respawn and beat the same enemy enough times, then you're set. Also, I got the upgraded weapon before the first one, so when I got the first one I was then downgraded. That probably shouldn't happen. The first should always give you the slow shot, and the second should be the high-speed weapon, no matter which you obtain first.