If i had to choose one set of dies for 300 aac blackout this would be it. - RCBS Small Base Taper Crimp Die Set - AR Series

If I had to choose one set of Dies for 300 AAC Blackout this would be it. Right now I have this set of dies with and extra RCBS T/C Seater Die I keep the two RCBS AR dies set for the 2 most common bullets I seat in 300 Blk the Sierra #2121, and the Sierra 220gr. HPBT MK.

I use a LEE 3 die set for the first forming of brass after I cut the neck off in the jig then debur & chamfer the fresh cut brass in a Hornady Case Prep Trio, after that I spray the brass with Hornady One Shot Dry Lube let it dry. After that I run the brass in the LEE size die for it's initial forming and sizing, I then run it again through the RCBS FL AR Size Die to move the last of the brass into shape. Trim it to within 3-5 thousands of its final size with a WFT 2, and the final trim is done on and RCBS Trim Pro 2 with 3-Way Cutter. Once the brass is trimmed the Crimps get taken out with and RCBS Primer Pocket Swager 2, or RCBS Military Crimp Remover Small. I use the RCBS Flash Hole Uniformer, and RCBS Primer Pocket Uniformer as well. Then the brass goes in the ultrasonic cleaner gets rinsed and dried and it's ready to go.

It's a lot of steps not all are needed that's just my method for making 300 Blk brass. I like the 2 different size die approach because the the RCBS AR Die moves so much brass around at once for a single forming. I find I get better brass forming it with the LEE first then using the RCBS for moving that last little bit of brass into place. You can feel the press moving brass with both dies the RCBS really gets a nice shoulder on the brass and shifts more of the brass to the base of the new shell.

For once fired 300 Blk I use the RCBS die to reform I see no need for the two step forming with once fired brass.

Make sure to follow the setup directions on whatever dies you use remember for size dies it should contact the shell holder plus ¼ turn to ½ turn more and just about any die that's made right will get your brass within SAMMI spec's. Remember to clean sizing dies and check the vent hole at the neck/shoulder. With 300 AAC Blackout it's not as critical the shoulder and neck are short however on 5.56 NATO a dirty size die will give you dimpled shoulders as well Running the brass into the press to fast for air or lube to escape something has to give and the shell is the weakest out of all the forces involved.