Thank you, Dolomite_supafly!

[Hozzie]

Be aware that some Lee 300 Blackout dies are out of spec. They do not size the waist down enough to chamber resized cases in some guns with tighter chambers, like Daniel Defense.

To check the neck measure the loaded round's neck. It should not exceed .335". When loading larger cast bullets the case neck can swell beyond .335". And then the rounds may not chamber or the fired round might have excessive pressure from the case not being able to release the bullet from the case neck.

Finally, trim the cases shorter that the max of 1.368". When trimmed to max length the case neck can be held closed on tighter chambers. It can also prevent the rounds from chambering. There is nothing wrong with going much shorter providing the bullet it held in place. I trim my cases at 1.35"-1.355" and have never had a problem. The cases headspace on the shoulder, not the case mouth, so having a short case is not going to be a problem. I had some cases I messed up on and were trimmed to 1.34" or smaller. I loaded them like the properly trimmed cases and they shot fine.

 

Thanks Gordon.  My loaded rounds are at .332-.333 and the fired rounds are .335, so that appears to be ok.  I am going to check my case OAL.  I cut them about 1.355, but that may still be a bit long, or my die wasn't down far enough.  I can easily try to rectify that and test again.  Thanks for the feedback.

[Clod Stomper]

They seemed to chamber ok, but may have been a little tight.  I loaded the rounds to 2.245 so just under the max oal.  I think I just need to do some testing with various things and figure out what the real issue is.  I am sure I will figure it out. 

 

One issue that I have had when forming cases is not setting the shoulder back far enough.  Then they won't chamber.  Even making sure the die is touching the shellholder, there is enough "spring" in the brass to keep the shoulder from setting back enough.  I had the same symptoms that you're having.  I've experienced this with forming 7.65x53 from .30-06, .300 Savage from .308 Win, and 6.5 Grendel from 7.62x39.

 

Ultimately, I had to shave down a shellholder to get the shoulder back far enough.  

 

I haven't formed any .300 Blackout brass yet, so I don't know if this is ever an issue.  But it may be something to consider.  Especially if you try some factory brass and don't have the same problems.

[mikegideon]

I don't do AR's.

 

You should. They won't smack you in the mouth like a Mosin :).

 

Ditto on your sentiments, though. Thanks G! Just found mine in the mailbox.