[opaww]

When reloading most semi-auto handgun ammo you do not Roll crimp the case.

Roll Crimping cases is mostly for revolver handguns.

Known cases not Roll Crimped these cases are taper crimped.
.380auto
9mm
.40S&W
10mm
.45auto

Known cases with Roll crimp
Any long colt
.38 spl.
.357 mag
.41 mag
.44 spl
.44 mag

Roll Crimping causes an increase in chamber pressures sometime it is minor and some times it is not.

Taper crimping helps keep the bullet from driving back into the case with auto handguns

Corrected as per robocop

[robocop10mm]

Not exactly.

There are two main kinds of crimps for handgun ammo, Roll and Taper.

Roll crimps are for revolvers and taper crimps are for auto pistol rounds. A taper crimp basically negates the flaring needed for proper bullet seating and very slightly reduces the mouth diameter to hold the bullet firmly. Uncrimped, the bullet will tend to "set back" or get pushed into the case mouth when chambered. THIS will greatly elevate pressures from compressing the powder charge.

[opaww]

Agreed....taper crimps as slight and not very noticable so I just think of them as no crimp myself

[robocop10mm]

I don't want to give the wrong impression to newbs that may view this. No crimp can be disasterous in the terms of feeding reliabilty and pressures.

[opaww]

Agreed......

[noylj]

With auto pistols, the main forces on the rounds is the bullet hitting the magazine on recoil, thus pushing the bullets into the cases. In revolvers, the main force is the bullets being pulled out of the case during recoil and binding the cylinder.
The only crimp any auto pistol needs is just enough to prevent the bullet being pushed into the case. This goes to the old "press the nose of the bullet into the edge of the workbench with moderate thumb pressure" test. That is all you need.
With a revolver, the more recoil, the more important the roll crimp becomes.
For an auto pistol, you need enough taper crimp to remove the belling. This was traditionally done by looking at the side of case against a brightly lit wall to be sure that the case mouth showed no flare and you would also run your finger along the bullet/case interface to be sure there wasn't enough bell to catch your finger.
Now, everyone wants to measure stuff so you take your case diagram (say .45ACP) and look at the the case mouth dimension (0.473" for .45ACP). Your taper crimp should be +0.001 to -0.003" (0.474-0.470" for the .45ACP). All you want is to iron-out the case mouth and have the round pass the "thumb" test and feed in your gun.
A roll crimp needs to be applied to a cannelure or groove in the bullet. Again, you don't want too much or you will distort the case and lose the bullet-to-case tension and have a case where the bullet is only held in by the crimp and the case could be so distorted that it won't chamber.