Re: Hand Gun reloading
What koodbe said above on straight wall semi-auto ammo (.380, 9mm, .45 ACP, etc). These rounds headspace off the rim of the case mouth - the tip of the case mouth hits a ridge inside the chamber which keeps the round from going any farther. If the round goes in too far, the firing pin may not set off the primer. Or worse, the round will fire but the case is so far away from the bolt face that it ruptures. Not pretty.
After belling the brass prior to seating, it needs to be brought back down in diameter. In any die set for this type of caliber I've ever used, the seating die has a built-in taper crimp that you adjust by screwing the die in and out. Sometimes if you bring the brass back to where it looks and feels snugged up against the bullet, it is actually too small in diameter. You need to check the diameter at the rim of the case mouth and compare it with the specs from your loading manual.
For instance, a .380 gets its name from the fact that this is the optimum diameter of the case mouth. It's the same diameter for the 9mm. Most ammo looks a little funny if you set the case mouth to exactly .380 - the case lip forms a ridge that sticks out away from the bullet. Shoots fine but looks weird, and that ridge could cause the rounds to hang up in a magazine, etc. You can get away with a case mouth that is a little smaller than this. In fact, most factory ammo I've measured is a little smaller than the specs call for. It's still going to hit that lip in the chamber unless you get it small enough to miss it all the way around - too small by twice the thickness of the neck walls. You can make up a dummy round and test how it feeds in your gun(s). If it comes to a solid stop when it is seated in the chamber, you're GTG.
For rimmed ammo (.38 Special, .357, etc) The headspace is set by the rim so the neck diameter isn't so crucial. Most die sets for these calibers have a roll crimp built into the seating die. For a light .38 round, not much crimp is needed. Too much will just wear out the lip of your brass early. For magnum loads in revolvers, you need enough crimp so that the recoil from one round doesn't cause the bullets in the other rounds to stretch out from where you seated them - and lock up the revolver. A good crimp is also needed for ammo going into a tubular magazine - lever guns.
I loaded thousands of pistol rounds in a single stage before I moved to a progressive. There are a lot of things to watch for and it's hard to watch them all if you're just learning. If you go to a progressive, make sure there is any extra hole for a powder checker or lockout die. Unlike rifles, it is real easy to double charge a pistol round and never know it until you pull the trigger. This is even more of a risk if you start out on a progressive.