Got it fixed!
Turns out there was a bit of a metal burr in the recess where the slide pin bracket is supposed to snap in to. Just had to grind that down a bit, and it went right together.
Thanks for all the help guys. And next time, I will completely disassemble the gun and put it in a box before shaking it until its assembled. If that doesn't work, I'll just toss it out the window.
