Righteous Fondue

I've been looking at the existing magazine design and I'm wondering if it would make sense to use metal/plastic U-channel to make the magazine bodies from? 
I'm not sure if you'd be able to make them as thin as you are with 3D printing, but you could just cut two segments of U-channel for each magazine, then you'd only need to 3D print/resin cast a small amount of material for the bottom spacing cap, follower, and top dart lip. The u-channel could be easily and consistently cut in a jig with a bandsaw.

McMaster sells U-channel with 9/16" inside width (part number 4592T26), and wall thickness of 1/16" in aluminum, which seems to be the thinnest possible giving you a mag thickness of 3/4", while still giving a bit of wiggle room inside that .5" interior width u-channel might not allow for.

I don't believe that drain blasters have an OPRV, at least none of the ones I've seen do. Barrel lengths for drain blasters can be up to 4 feet, I use a 4 foot PETG barrel on my blue cobra and that seems to get optimal results.

So thats a confirmation that the current 2k pump will work?
Also, do i just shove it in and hotglue/superglue it in place?

You really should attach some Vinyl tubing to the 2k pump then attach that to the tank, since it will be easier to get leak-free than a hard plastic to plastic bond. Also if something breaks or leaks then it's a lot easier to just add some new vinyl tubing than cut and re-bond two pieces of plastic. 

I'm a huge fucking weeb who cosplays and builds props

Here's my cosplay of Guts from Berserk:

I was at Fanimecon in San Jose, CA and met a Griffith Cosplayer
I'm the dude in the black armor 

I also do woodworking:

Made a stool out of mahogany and mahogany-family woods (don't remember some of the exact names) using no metal, just wood joinery.

Made a Shoji Lamp:

And I'm pretty handy with a laser cutter, made a batch of business cards for an art show:

And another related thing (sorry for pic spam mods) I make wooden rings:

I'm actually trying to get back into nerf a bit, fell out for a while but with summer coming up I'll hopefully have time for it, and will try to make it to some local events in the Bay Area.

Slush-cast cure time varies depending on the resin, but if you let it set overnight you'll be fine. Its a matter of hours, not days.
 

Casting a half-shell at a time is possible, but could be messier, or more difficult, as the resin is more likely to spill out. It shouldn't be too hard to cut the shell in half if you leave some tabs or indents to indicate where to cut with a dremel.

The trigger and priming bar could be difficult, especially the trigger since there are some very specific pieces holding it in place. I'd say you can just mark half the thickness of it on each side, and cut a slot out, but it varies depending on the blaster.

Priming bar shouldn't be hard if you leave a cavity/indent of the slot for it, just make sure the internals you put in are centered.