Rate of fire
The rate of fire in a gun is less about the gun itself than the training of the shooter. If you give a good shooter a bolt action rifle, he can kill more people than a neophyte with a fully automatic machine gun. In most situations, if you give an experienced shooter either, the death count rarely would change much significantly.
The premise of magazine limits is that somehow in the fraction of a second while a shooter is reloading, that you can cover a 20-30' gap and take away their gun and subdue them. This leads to ideas like 10 bullet limits (since the first 11 dead people don't count), no removable magazines and other stupidity. Anyone who has ever watched tactical shooting drills knows better. Watch any competent shooter or gamer do reloads. Whether a shooter has to reload 0 or 10 times in a shooting spree makes no real difference to the deceased: shooting sprees end by surrender or suicide (or getting shot), not by takedowns during reloads. And that's before realizing that if you can't keep semi-automatic weapons out of criminals hands, magazines are even easier to make or buy.
Mass murderers have used lever action, pump shotguns, fixed magazines, 23 used revolvers and one of the worst used a bolt action rifle -- there's no correlation between death tolls and magazine limits or even action of weapon. Anders Breivik is still responsible for the largest mass shooting (77 dead, 319 wounded) in Norway (with some of the strictest gun controls in the world). But in the U.S. the largest ones not carried out by government include a bolt action rifle, a pistol, and of course bombs (Oklahoma city) and box cutters (9/11). You don't need to fear the tool, you need to fear determined sociopaths that are lured to infamy by the media. Limiting guns, can just make them more determined to use more effective means. But that's a topic for another article.
This article shows how easy it is to bring firepower to bear, and how quick it is to reload, in many forms. Some will claim, "but they're an experts"... while that's often true, the difference between competent and the best, is often a second or fraction of a second -- so it doesn't really matter. There's a reason why no one has successfully rushed a mass shooter during a reload. Remember, you don't know when they're going to reload, thus by the time you've figured out they're reloading, they've already reloaded. Shotguns and lever action rifles can be more effective than that, as you can just keep adding rounds as you're shooting.
So the problem is gun-users know better than the gun-controllers. And the informed are not resisting because they like their murder-toys, they resist because they know what won't be effective. If gun-controllers were more rational, they would ask the experts, and listen to the answers. Some day, I'll meet a gun controller willing to consider the other side -- but I've only been trying to find that person for 40 years now.
Here's Marshall Luton hitting multiple targets with tactical reloads to illustrate the point. Imagine trying to rush him while he's reloading (each ring, is him hitting a different target).
And while he is very fast, and professional, this is a skill that many people practice, and can do well.
Before there were removable magazines, you put bullets were held together with a "clip", so you could push the bullets into a fixed magazine, all at one time. This kind of system predates metal cartridges, and goes back to the 1700's. Here's the 1930's era M1 Garand that was standard issue in WWII, or Soviet SKS from the same era. Both these guns are Curio's & Relics, meaning they're antiques (greater than 50 years old), or historical guns to collectors -- which means they often have different / easier rules to buy/sell/gift. Some M16 are about to go into that category.
Here's a competent shooter, showing how fast it is to reload with these older guns. You push the bullets in, and they're ready to fire (no removable magazine necessary).
California keeps trying to harass shooters, so they try to outlaw removable magazines for "Assault looking rifles" (the AR platform). This means if you have a pistol grip and collapsable stock, that you must take apart the gun before dropping the magazine, or use load assist device like above. But if you have the same gun (or a more powerful one), that doesn't have a pistol grip, you're fine. Showing that these laws don't change firepower. Quite a few companies have solutions on the market, that are as fast as having a removable magazine, but are "compliant". While they're effective, every time someone has to use a work-around for California fascist, they curse their government and the voting public that is ignorant and malicious enough to pass these laws.
This isn't just a problem with semi-automatics, it's not practical to shoot someone reloading using a revolver. Remember, 23 mass murders have been done with revolvers, none were disarmed while reloading. Watch World Record holder Jerry Miculek, reload. 
There's two topics with pump actions, how fast can you shoot, and how fast can you reload.
How fast can you shoot, is basically as fast as a semi-automatic, there's virtually no difference in firepower. Here's Patrick Flanigan throwing 6 clay pigeons in the air, and hitting them before they hit the ground.
While Jerry Miculek shows his reloading techniques, using speed loaders in a shotgun. While he is using a semi-automatic shotgun, the same loading mechanism applies to a pump action. It's arguably faster than dropping and replacing an external magazine.
A lever action rifle is even worse. Not only can people fire nearly as fast as a semi-automatic, but fixed magazine weapons like this, allow load-as-you-go. (You can fire and drop rounds in while walking, and never run out of ammo). He's only popping one extra one in at the end, but you can see how blindingly fast it is.
Pre-WWI the British and the Americans had training with their bolt action rifles, that included a mad minute, seeing how many holes you could put in a target at 300 yards using their bolt action rifles -- you had to do 15 to qualify, but the peak was about 38 shots per 1 minute. But that's 300 yards with a really powerful round (30-06), if you reduced the range and size of round to something about 1/3rd the size (a .223) and you could easily up the rate of fire.
Even a bolt action rifle can fire faster than anyone can charge them. Here's a guy just screwing with an 1890's enfield, showing how fast he can shoot.
Emergency reloads drop you to a slightly slower rate of fire, to about 1 per second, but you still aren't charging anyone from any distance. And remember, many bolt action guns have internal magazines, so you're back to speed loaders (stripper clips) and dumping 10 or more rounds in, at once.
Charles Whitman, DC Sniper.
Mitigation factors like magazine limits, "assault weapons", or even barring all semi-automatics does NOTHING to stop or slow mass shooters.
- All these guns have easily exchangeable parts, and the parts aren't illegal to buy or own, installing them is. So these guns have to be modified to make them compliant with California law, and it'll take 5-15 minutes to put them back on, with off-the-shelf mail-order parts.
So the law can ONLY harass those that obey the law, and mass shooters have a tendency to break laws. And even if you complied with the law, it does virtually nothing to change the effective rate of fire or lethality for the weapon, it only changes the cosmetics.
Imagine California did the same to cars. They'd require all red cars with a spoiler to have a block under the gas pedal, so you could only press it half way (or you vinyl wrap the car in a different color), under the theory that would help with speeding. Of course it doesn't, and if anyone wants, they can remove the pedal or the vinyl wrap. And that's what it's like to be a gun owner in California.