So, I'm trying randomize a gun's sprite in a similar manner to the Ronin's heads and stuff like that, and I tried that same script, but as I've found out, held devices are hard coded to default to their first frame of animation, so I'm wondering if there's something like a Self.defaultframe I could try and place into the sprite randomizing script that will fix this.

As far as I know, there's no way to modify the sprite directly or mess around with an HDFirearm's frame using Lua.

You could try using an attachable that has the different sprites and the gun is a plain rectangle or something, just to serve as something to get shot at.

There isn't any trickery needed to change magazine sprites, so that part is at least easy.