So if you find yourself trying to grind for parts/weapons I have a tip or two:
The second mission on code red is a single area. All four item chests are within ten seconds of starting. You can know your drops almost immediately. And whether or not it's worth killing the enemies to earn them or just retiring and relaunching the stage. This saves a lot of time.
It's also a really easy level. Just zombies. So if you do want the items, it's very easy to finish the level quickly. I recommend a shotgun for CQB and an automatic with a fire status effect (this is to burn through shield enemies) and things go pretty smoothly. Especially if you're high level (don't need to be recommend, since you're not playing for experience -- don't have to kill the last enemies either, and you can heal if you need to -- far as I can tell be above rec lvl doesn't seem to affect drops).
Obvioulsy this is not the funnest way to play the game, but if you're trying to get better weapons quickly or upgrade parts, seems to be working for me so far.
As far as quality of the drops... I haven't seen a purple or better yet, but I do get a lot of reds, and have gotten lvl 100 weapons. Purples are rare to begin with, so they maybe drop here, I just haven't seen it yet.
Auto-shot still eludes me. :/