The 1994-2004 law on so-called "assault weapons" prevented the manufacture of new detachable magazines for firearms with a capacity of more than ten cartridges. This forced the redesign of magazines for almost all semi-automatic firearms. Ever since the Tucson shooting just over a year ago, when somebody used a Glock 19 pistol with a 33-cartriged magazine designed for the Glock 18 submachine gun to kill six people and injure several others, some people have been calling for this ban on "high-capacity assault clips" to be put back in place. However, the standard magazine for that pistol, which is also owned by Representative Gabrielle Giffords, can hold up to seventeen cartridges. The vast majority of detachable pistol magazines hold at least fifteen cartridges, while the vast majority of detachable rifle magazines hold at least twenty cartridges.
The first time that the ban was in effect, it had no effect on the violent crime rate or the crime rate with firearms. In fact, the most popular firearms for use in crimes hold eight cartridges at most. They tend to be small-caliber revolvers or pistols with single-stack magazines, along with the Mossberg 500 shotgun.
Use of 33-cartridge magazines actually makes it harder to commit a mass shooting. Above-standard capacity magazines are far more likely to jam than standard-capacity magazines (as are the below-standard capacity magazines that the 1994-2004 law mandated). The only reason that the Tucson shooter (whose name I will not use, so that he may hopefully rot in obscurity) was able to be subdued was because he was in the process of clearing a jam. Clearing a jam takes a lot longer than switching magazines. Had he used two standard-capacity magazines (one more cartridge than would be in the magazine that he used), he would have been much less likely to jam, reloading would take less than a second, and he could keep one cartridge in his weapon's chamber while he was reloading so that he'd still be able to shoot.
Over the past year, the Brady Campaign, in their usual dishonest manner, has started calling all magazines that aren't significantly below standard capacity "[Tucson shooter]-style assault clips." I'm not going to discuss the difference between clips and magazines again. However, the "assault" part is especially misleading. Above-average capacity magazines, such as the 33-cartridge magazine that was used at Tucson, are primarily designed for target shooting or specially-designed automatic firearms that would be less likely to experience feed issues and jam. Therefore, they are overwhelmingly not designed for "assault."