300 B/O...

[Jody Simcox]

I’m about to start loading 300 Blackout for my sons 10.5” AR pistol...I’ve only loaded for bolt actions in the past...in prepping brass prior to seating bullet, is it ok to just expand the case neck with a mandrel or do I need to flare the case mouth, seat, then crimp? Thanks

 

[Jager]

For semi-autos like your son's AR pistol you're going to want to full-length resize your brass.

You'll generally want to chamfer the mouth of the brass, but you don't need to flare it like you would for straight-wall pistol cartridges.

A crimp may or may not be necessary, depending upon how much neck tension you achieve in your sizing process.

Have fun. The .300 Blackout is a fabulous round!

 

[Jody Simcox]

BTW...I have Dillon dies (seater and taper crimp), but was maybe gonna load these first few on my single stage press- my 750 isn’t set up just yet...is it even ok to try that? Just loading some subsonic plinking rounds with cheapo plated 220’s...

 

[Jody Simcox]

For semi-autos like your son's AR pistol you're going to want to full-length resize your brass.

You'll generally want to chamfer the mouth of the brass, but you don't need to flare it like you would for straight-wall pistol cartridges.

A crimp may or may not be necessary, depending upon how much neck tension you achieve in your sizing process.

Have fun. The .300 Blackout is a fabulous round!

I don’t have a FL sizer just yet...is it necessary to size before loading? I was planning on just checking every piece with a case guage....new B/O brass from Starline...thanks

 

[HandgunHTR]

My advice is to reload them exactly like you would if you were loading for a bolt action when using new brass. In regards to crimping, when I load the subsonics, I don't crimp. For the 150s and 125s, I do use a Lee factory crimp die. Not because I am afraid of the bullet moving, but because with the lighter bullets I seem to get a more consistent powder burn (as verified with a chronograph).
After the first firing, you will need to full-length size the brass to ensure to limit the chances of a failure to chamber.

 

[Hoser]

For semi autos FL resize all your ammo. Bump the shoulder back 2-3 thou.

 

[Oso]

As everyone stated... FL Resize is a must for AR 15's. What is your brass source... new factory, range pick-up, or you had fun forming it from 223 cases? Regardless, the answer is FL Resize.

With that short length barrel, H110 will be your best powder. Run of the mill dies, such as Hornady New Dimension, work fine. You don't need to use a separate expander mandrel die. I find a tapered expander in best to smoothly open the case mouth (removes the flat spot that occurs as result of case hitting shell deflector on AR15.the ball expander. With the right neck tension I don't feel the need to crimp the bullets.

Check out Johnny's reloading bench. He did an in-depth series on the 300BO.

 

[kindabitey]

BTW...I have Dillon dies (seater and taper crimp), but was maybe gonna load these first few on my single stage press- my 750 isn’t set up just yet...is it even ok to try that? Just loading some subsonic plinking rounds with cheapo plated 220’s...

Might be a good idea to treat the plated 220s as a coated lead bullet. It won't hurt to flare the neck a smidge
and add a light crimp after seating. A lee factory crimp die works well for this. It's pretty much a collet crimping die.