Follow the pictures and you'll get through A-OK! d(^^d)
This isn't really that hard to make (aside from the Stock, but you can do pretty much whatever you want with that).
Anyways, now to the pictures.
I give full credit to Brad/DGM for the trigger mechanism.
Step 1: The Front Half
Step 2: The Back Half
Step 3: Connecting the Halves and Finishing Up
Step 4: Loading and Firing
Elementary, my dear Watson!