This can be used as a standalone display, or as an insert in Whity's “150x100x50” Rugged Box. https://www.printables.com/model/258431-rugged-box-parametric/files

Holds 50 rounds. 
Printed in PLA, with a .4mm nozzle, 20% infill, standard quality.
As long as your printer does not Z-band too bad, the rounds will slide in and out with little to no effort.

I included a small storage space for oddballs… Markers/pens, ear-pro, small note pad, etc...
If used in the Rugged Box, your max overall cartridge length is 57mm (if it fits in an AR Mag, it fits in the box).

Why this design?
I wanted something both practical and beautiful that could take advantage of Whity's Rugged Box.
This may not be the most space efficient designed, but I think it manages to maintain enough utility to justify it's ‘eye candy’.

Note: A good first layer is important, ANY lifting will warp the slots and make the tray useless.
Glue-stick is your friend!
My Ender 3 printed this model well, but I included a test slice file if you want to check fitment for your box and/or rounds.

Made 100% in Tinkercad

Printables Tags: ammo; reloadingtools; reloading; 300blackout; ammostorage; ammobox; 300bo