Getting the outside of the cases clean is just a matter of polishing them up with metal polish, Brasso, Duraglit or similar. If you need something a little more abrasive to really buff them up use Solvol Autosol.
Wire wool will remove any green deposits if they're really scruffy.
Ceaning Inside is more important for proper function during firing.
Patience is the key here mate, you'll need something to poke inside the cases around which you can wrap some cloth or something. Get the inside as spotless as you can.
If you happen to have any Airsoft guns, you may have a cleaning rod that has what looks like the eye of a needle at one end? Push a bit of rag or something in the 'eye', wrap it around a turn or two then wipe and polish inside the case with that.
I've got a 'write up' loading procedure for Marushin MP40 rounds that are esentially the same as the M16 ones. I'll dig it out and post it on here. There are'nt any photo's unfortunately but if you read it whilst looking at the rounds its dead easy to follow.
Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.