For when 20 isn't enough, here are the shell pieces for Alice Coatducks Vorpal magazine, extended to 30 rounds.
Over 360mm long this can be printed at 45° on a 300x300 print bed (But only just)
This version of the magazine is not compatible with rectangle profile springs and MUST use the drum spring as the Vorpal mag was originally designed.
I had A LOT of trouble printing this, as the vorpal magazine requires a clean first layer print to ensure that the resultant magazine feeds reliably. One of my prints was not entirely flat once it came off the print bed. I hope yours comes out well!
This is for use with the Vorpal magazine:
https://www.thingiverse.com/thing:3097776
And DKDavids version of the feed lips found here:
https://www.thingiverse.com/thing:3122092