Originally Posted by locosway
For UOC you can have the magazines anywhere that doesn't make the firearm "ready to fire", meaning not inserted into the magwell or attached to the firearm. So, you can have them in your pocket, under your belt, or you could pickup a cheap mag holder for your belt which is my recommendation.
I wouldn't keep an unloaded mag in the firearm unless you're trying to give the appearance of loaded, which may be an intent if you're carrying to ward of unwanted persons. Keeping the magwell empty allows for a quicker loading time, and keeps the magwell free of any possible jams or obstructions that could be caused by trying to drop the empty mag and load the full one in a time of high stress.
Magazine is an essential part of the firearm and therefore same concealment rules apply - you cannot put magazine into your pocket while UOC, you need to keep it visible (People v. Hale 1974)
As for keeping empty mag in the well - it reduces your loading time, but keeps some dirt out of gun too. So you decide if you want to plug that hole or not. I guess it would depend on where you carry.