I just got my RCBS taper crimp die for the .300 BO.
When I started loading lead/plated bullets I found out the Redding seating die was just a bullet seating die, no ability to get rid of that slight case mouth bell I'd put on them for the soft flat based bullets.
I was bored, so I started reading the instructions. I ran across a couple/three things that made me curious.
One was the instructions say the RCBS taper crimp die is beneficial for crimping if the cases are not of uniform length. Then, a sentence or so down the instructions recommend you have cases of uniform length for more consistency in crimping....
The instructions also mentioned the taper crimp die is really good for bullets with a canelure so you can crimp the case mouth into the canelure.
If I get a chance in the next few days I might try to load some .300 BO and see how well, or not, it works. All mine are trimmed and I have no intentions of using bullets with canelures, I just need to get rid of the bell at the case mouth.
If it works well enough I might order one for the .357 SIG (since I had an issue awhile back while loading up the left over brass - varying case length resulting in some bulging of the shoulder when the round went through the crimping die.