Magwell

The magwell is the opening in the pistol grip that takes the magazine. It matters in two moments of a reload, and both of them are about time. The first is the release. The magwell has to point straight down when the thumb hits the release, so the empty magazine drops free under its own weight. An empty live magazine is light, and if the gun is tilted it can hang up in the well without the shooter noticing until the fresh magazine arrives and runs into it. The second is the seat. The mouth of the magwell is the point the eyes lead the fresh magazine into, and it is also the point where the eyes are done: the moment the magazine catches, they go back to the target and the support hand finishes the seat blind. Watching the magazine all the way home is the most common way to lose a fraction of a second on a change that was otherwise clean.