300 Blackout headspace issue...

[wscywabbit]

So I have a friend that built himself a 300 AAC Blackout AR this winter. Its a great round, lots of fun and he and I have learned a lot from the build. Since this is a fairly new round (Only been SAAMI spec approved for a couple years or so), I got him the new 300 blackout dies from Lee for Christmas.

Of course, he has never done reloading, and has spent a small fortune to get setup to cut, form and load his own brass. (Merry Christmas, you're welcome LOL). This weekend we took several small batches of his reloads to the range to run them through my chrono and test them for function... which is where we found our first issue!

It seems that some NATO brass is a little bit thicker than others, and he ran into chambering issues with brass with the headstamp S&B from the Czech company Sellier and Bellot. The problem is that the 300 AAC is made by cutting the body off at the shoulders, then necking the body down to .308 and trimming to length. With the body of this particular brass being thicker, the OD on the neck would stretch out of spec when the bullet was pressed into place. Luckily, when it was ran into the chamber, it would not seat all the way, and jam the carrier group just out of battery... Luckily we never got far enough into it to actually set one off and have a really bad day.

He thought is was a problem with the die set, until I explained to him what was going on, and that he needed to check each headstamp type with his calipers to make sure that they are in spec. I figured I would throw this info out there in case there are any others beginning to load their own Blackout cartridges!

[62chevy]

Not doing 300 Black Out but have read that some necks need to be reamed after forming to maintain them with in spec. so your friend now has learned something and won't make the same mistake again. been there done that with the learning curve on reloading.

Glad nothing bad happened.

[Ranch Dog]

I have not worked with the Blackout but I think you are right on target with the "flow" of the brass. I have noticed that S&B is thick in my 7.62x39 and they put a slight negative taper on their primer pockets that this tool will correct: RCBS Large Primer Military Crimp Remover

This tool will allow the Crimp Remover to be used in a drill or other power tool: Redding Powder Tool Adapter

I believe someone here on the forum pointed me to these tools and they have been a great help with this brass!

[Model 52]

I had similar issues way back in the day necking down 7.62mm NATO brass, which has thicker walls than commercial .308 brass.

He can still use any brass he chooses, but he'll need to turn the brass down to reduce the neck thickness with outside turning, inside turning, or both.

Outside turning should be adequate for his AR related purposes and the K&M tool is relatively affordable at around $80-$85 shipped and pretty straight forward to use.

http://www.kmshooting.com/catalog/with- ... older.html

Ideally you'd use a tubing micrometer to get the exact thickness you want/need, but a pair of calipers will get you in the ball park and you can also do it by trial and error, turning just enough to chamber your rounds reliably.

----

With that said, he'll still want to sort his cases by head stamp as the thicker brass is thicker throughout the case and will consequently have lower volume and generate higher pressure, so it'll need some tweaking of the powder charge to produce the same velocity as the thinner walled, larger volume cases.

[wscywabbit]

Thanks for your replies guys I'll let him know of the tools you mentioned. Right now he's just pulled all the S&B and tossed them. He had several different head stamps, including other NATO, and it was the only one that he had problems with.

I know he has a pocket swager from RCBS on the way, he's very excited about that!

[Missionary]

Greetings
Yep a case of the need for a neck reamer.
I first found this with my first caliber .243. there was no brass in Chatanooga so I read some more and started squeezing 308 necks down to .243... Discovered the first was a bit tight to chamber. The second blew the primer pocket loose and the primer fell out. Shortly after that 2nd round I bought a inside neck reamer and still have some of that origonal 50 pieces of 308 brass.
Mike in Peru