1. Acquire two of the same weapon.
2. Put one in your gun slot and the other in the hands.
3. Make sure the one in your gun slot has the magazine loaded while the 2nd one is in your hands and unloaded and you have no extra magazines in the backpack.
4. Exit inventory screen and press the R button to reload. (You should see the reload Icon before doing this since the gun in hand is empty.
5. This duplicates the magazine but they are linked. You can shoot and it takes bullets from the mag. You can also remove the mag and put it in your backpack so you can reload ammo for the first gun without having to actually switch magazines or disconnect it. To actually duplicate it as a separate mag continue following the directions.
6. Remove the Magazine from the 2nd gun and place it in your inventory space.
7. Log out.
8. Log in.
9. The Magazine is now fully duplicated and useable separate from the original and no longer linked as noted previously.

I assume this is possible with other weapons than just the .22 but I haven't taken the time to try duplicating it with them as well. I also think it's possible with the pistols and I've done it before but again I haven't retried it multiple times to confirm without a doubt that I can always do it.