What everybody else said!
Trim your cases to the same length. If you don't have a trimmer,
then buy the Lee Factory Crimp Die, and seat the bullet and
crimp in two separate operations. The Lee Factory Crimp Die
won't buckle your cases like that.
I always seat and crimp in two separate operations when I can.
It's slower, to be sure, but I get a more harmonious outcome......
Progressive presses with revolver or pistol rounds are the exception.
If the cases all measure the same length, here's how I do it:
1. Seat the bullet to the COAL required, or until the case mouth sits
in the cannelure. I like the case mouth nearly to the top of the cannelure,
as I believe it allows tha maximum amount of case to crimp into the cannelure.
Seat all your bullets for the batch.
2. Unscrew the bullet seater a ways - or just remove it entirely. Then
screw the die body down until it touches the case mouth, plus a quarter of
a turn. Crimp. Screw the die in or out until you get the desired amount of
crimp, then set your lock ring (without, of course, moving the die body.)
Then crimp the rest of your cartridges.
I've never found an IDEAL result with EVERY cartridge doing seating and
heavy crimping in the same operation. Mild taper crimp, no trouble. Heavy
roll crimps for magnum revolvers or levergun cartridges I've always found
work better in a separate operation.
Let us know how you get on!
-Stretch