What you'd need.
I start by clipping off the sights making sure to leave a little bit of the rear casing.
Next chop the entire barrel off. Make sure to hold it so it doesn't go flying into space. You'll need it.
The trickiest part of this is getting the wings off. Make sure to use a new-ish blade and carefully shave it off. Watch out for those fingers. It doesn't need to be perfect right now.
Also chop the bayonette lugs off.
Push the blade in at an angle to make a notch. I do this in line with the pinky finger on the hand.
Shave the top edges of the casing to begin the rounding. Make sure to keep the notch that was made a hard edge. I shave one line, not worrying about roundness yet. If you look at the weapon from the top and break the casing into thirds, you're aiming to keep the center 3rd untouched and the other 2 are your shaved angles. That goes about the same looking at the sides but it's slightly taller. Continue to refine the edge until it's round. I do this by lightly filing and some x-acto scraping.
Chop the barrel to remove the little notch. Again make sure to hold it while you're cutting. It might need a little filing to make sure it's flat.
Next take an X-acto or something sharp to plot out pilot holes. This could be whatever pattern. I do 3 holes on each side, then 2 on the top in line with the gaps between the 3.
Once that's set drill out shallow holes
That's it for the gun. Next up is some optics. You could use whatever you like, or just leave it flat like the picture.
Take an IG heavy weapons optic. I use the squared glass end so make a triangle notch just behind the centerline. I use a round file to do most of the work.
Chop the other end off and continue shaping
That's it. Probably the most time consuming part of my plastic stormtroopers.
I'll try to continue this with a full stormtrooper, but the rest is pretty straight forward GS work.
Here's a completed Stormtrooper