If a firearm has e.g. a seven round magazine, many people will load the magazine with seven rounds, insert it, and chamber a round, leaving the magazine with six. On many (but not all) firearms, it would then be possible to remove the magazine, insert another round, and re-insert the magazine. I've not studied the exact dimensions involved, but I believe inserting a fully-loaded magazine into a firearm whose action is closed will push down somewhat on the top round; a magazine which didn't have to allow that when fully loaded could be made a little shorter than one which did; not all magazines allow that, and consequently not all firearms can usefully hold "full magazine plus one".
Personally, I would tend to think that such loading would be a nuisance, since every time the weapon is cleared one would have a cartridge one would have to store somewhere other than the magazine, but I would not begrudge anyone who wanted to go through such effort in order to have a maximally-loaded weapon.
I can see how such a situation could cause the malfunction we saw here. Zimmerman's gun (Kel-Tec?), reportedly, did not chamber the second round. If he crammed it to get the extra round, the auto-feed may not have worked properly, thus giving him only one shot. Let that be a lesson to everybody: You don't need the one extra round that badly!
When you load seven rounds and put the magazine in you have the exact same situation as you have AFTER you rack one round into the chamber and then put the seventh round back into the magazine. The only difference is that you have to MANUALLY rack the slide to load the first round rather than the weapon reloading automatically. I have always loaded the extra round when target practicing and never found it to cause a problem. A seven round magazine is designed to hold seven rounds, I don’t know of any reason that a round in the chamber makes any difference.
Exactly. My 1911 is 8 in the mag, 1 in the chamber, cocked, locked. Why anyone would not have one in the chamber and a completely full mag is beyond me.