video fo an m2 being taken apart, the m15 is the same but split
Hey bud. Good luck with this project, sounds really interesting and fun.
In terms of design, both are the same essentially. Obviously Aluminum will cost you more, but generally Al machine precision is pretty good so you're very likely to get a nice case. However, it will ultimately depend on how many batches you make. Plastic is not really that cheap to produce just one case, but rather when you produce something in massive batches. Either way, unless you own a shop yourself or have good access to one, it will be relatively expensive. Just make sure when you submit the design to a machine shop to have it produced it's correct. Small offsets and incorrect tolerances, especially with aluminum, are really uncool. Metal is hard to make up for small design flaws, as it will always show. Also, keep in mind that aluminum dents, whereas plastic doesn't. Also, plastic is stronger than aluminum, just not at elevated temperatures, but can be more brittle. You get the point.
Tolerances should be similar. Ultimately the plastic will be molded in a mold that is cut out, so the machine precision on the mold cutter is what will ultimately determine the quality of your mold. Both will be equally good, probably down to the 0.001", so that should be sufficient. You can always pay for more though.
However, on an important note, if you are going to incorporate spring barrels, then keep in mind that this may jump the price up.
Just get a micrometer caliper and measure, but be accurate. Even better, have a rapid prototyper scan your board for a 3D mockup that is very accurate. I'm not sure what your budget is, but you're better off if you can find some others to put in so you can get atleast 10 cases built. Otherwise, I don't see it to be economically feasible at all. 1 prototype can easily cost $300 and 50 cost around $400-500 so it doesn't make sense to just make one or two. It's the cost of programming the machines to make your parts and stuff that generally takes up most of the labor, and the cost of the tools and molds etc. and of course machine time.
