Making magazines which work isn't all that simple. First, you have to have the proper dies made. Second, you need a skilled press operator. Gauges must be made to ensure that the magazine conforms to the design. There are plenty of sorry aftermarket magazines out there and I'm sure our members can attest to this.
Angle of lips, space between lips are all critical. Follower and spring tension can't be overlooked either (the Germans will not only test a spring for its strength or compression, but also for its rate of decompression). Even if the magazine is made properly, once installed into the gun, it must sit just right if the feed rib is to strip a cartridge off each and every time (and here, the magazine spring can be critical since if it is too strong, it is pressure which retards the travel of the bolt carrier and if too weak, unreliable).
Better to adapt an existing and readily available magazine than to make an entire gun from scratch.
If TSHTF, I wouldn't want a SMG. It's too close and personal. Better to reach out, reach out and touch someone with a varminter. If you want full auto, go MA-Deuce.