Which is why I have never understood that you can get extended magazines. They only serve two purposes, target shooting and killing people. Any hunter worth his weight will not have to expend 30 rounds to kill one deer, and most still use the good ole bolt action that only carry's five rounds in a magazine. Are they really necessary for civilians to get their hands on?
Actually, magazines that hold 20 or 30 rounds are the most practical for defense. For a self-defense situation on the street, it's often not practical to carry multiple magazines or speedloaders along with the gun and its holster. For home defense, it's likely downright IMPOSSIBLE to have more than one magazine (the one loaded in the gun) when you take up your gun for defense, as it's doubtful that you'll be sitting in full tactical gear ready to go up against whoever's breaking in or shooting into your house or whatnot. Especially in a panic situation, it's very possible that you may expend all 6 rounds in your revolver or all 10 rounds in your California legal magazine without hitting or stopping the target, in which case you need to reload; if you just got out of bed in your skivvies and the guy in your house starts shooting back down the hall, now you need to try and run for the rest of your ammo when you run out because you can't even carry any spare mags. Having magazines with a decent capacity is the best and most practical option for self-defense.
This is why I found Joe Biden's comments on "Just use a double-barrel shotgun!" utterly ridiculous. His entire statement about it was ridiculous and downright dangerous (like suggesting that you shoot through the door, which will get you arrested and jailed, or that you fire both shots into the air to scare people away, which will ALSO get you arrested and jailed when those pellets land on someone's car or head), but having a weapon with only 2 shots to defend yourself, your property, and your family is plain dumb.
On the subject of ammo purchase restrictions, do you have a number in mind? On top of the ammo kept for the intended purpose of the gun (target shooting, defense, hunting, or recreation), anyone who properly practices will often expend hundreds of rounds in a single session. And practice isn't something you just do once a month or once a year. Not if you want to actually maintain your skills. And don't suggest "Practice less often." YOU may not care about my aim being shit, but I do.
The usual argument for ammo restrictions is that it prevents mass shootings. Keeping in mind how much ammo a single practice session can go through, look at how many rounds are typically fired in mass shootings: about 200 at the absolute most. The Virginia Tech massacre remains the deadliest mass shooting in the United States and only 174 rounds were fired, with less than 50 people actually being hit. Unless you plan on restricting ammo purchases to one or two magazines worth per year, you're not really going to put a dent in anything.