I can't imagine why they'd make a roll crimp on a 9mm die unless you were loading for a revolver using moon clips. That out to be a specialty die, not part of a standard set. The reason why is because the 9mm headspaces off the case mouth, meaning that the diameter of the rim of the case mouth is what determines how far the round goes into the chamber. (see the diagram posted above by dbs). The SAAMI spec for that diameter is .380.
If you load a round and taper crimp the flair back down until it reaches .380, there is usually a little lip left between the case mouth and the side of the bullet. Looks a little funky but it will work perfectly. If you keep adding more crimp until it looks nice and smooth like a factory round, you will find that your mouth diameter is usually less than .380. In fact, if you check a bunch of factory rounds, you'll find that most of them are also less than .380. This is because the thickness of the case walls at the mouth is often less than the 0.0125 it would take to get a true .380. ---- .355+(2 x .0125)=.380
If the case mouth diameter gets small enough to slip by that lip in the chamber shown in the above diagram, the round can go too far into the chamber - so far that the case head is no longer in contact with the bolt face when the bolt closes. The firing pin may not reach the primer. It may reach the primer and knock the round farther into the chamber as it goes off, leading to all sort of problems ranging from case head separation to a catastrophic KABOOM.
You can get away with a case mouth diameter that is smaller than .380 up until the point where both sides of the case mouth are able to slip by. How much under .380 you can get away with depends on the exact dimensions of your chamber. I'd rather have a funky lip on my rounds than run the risks of getting too small, but that's a decision we each have to make. It's alright to get smooth, just don't get carried away.
As an aside to some of the earlier comments, the amount of flair needed is just enough so that you can seat a new bullet easily. If you flair too much, the taper crimp will bring it back down again. All you're doing by flairing too much is putting excessive wear on your brass, probably shortening the time before it gets work-hardened to the point of splitting. OTOH, a little bit more flair will make the bullet less likely to bulge off to one side in the case like it sometimes does if it's not quite centered when the seating begins.
If you load a round and taper crimp the flair back down until it reaches .380, there is usually a little lip left between the case mouth and the side of the bullet. Looks a little funky but it will work perfectly. If you keep adding more crimp until it looks nice and smooth like a factory round, you will find that your mouth diameter is usually less than .380. In fact, if you check a bunch of factory rounds, you'll find that most of them are also less than .380. This is because the thickness of the case walls at the mouth is often less than the 0.0125 it would take to get a true .380. ---- .355+(2 x .0125)=.380
If the case mouth diameter gets small enough to slip by that lip in the chamber shown in the above diagram, the round can go too far into the chamber - so far that the case head is no longer in contact with the bolt face when the bolt closes. The firing pin may not reach the primer. It may reach the primer and knock the round farther into the chamber as it goes off, leading to all sort of problems ranging from case head separation to a catastrophic KABOOM.
You can get away with a case mouth diameter that is smaller than .380 up until the point where both sides of the case mouth are able to slip by. How much under .380 you can get away with depends on the exact dimensions of your chamber. I'd rather have a funky lip on my rounds than run the risks of getting too small, but that's a decision we each have to make. It's alright to get smooth, just don't get carried away.
As an aside to some of the earlier comments, the amount of flair needed is just enough so that you can seat a new bullet easily. If you flair too much, the taper crimp will bring it back down again. All you're doing by flairing too much is putting excessive wear on your brass, probably shortening the time before it gets work-hardened to the point of splitting. OTOH, a little bit more flair will make the bullet less likely to bulge off to one side in the case like it sometimes does if it's not quite centered when the seating begins.
