Case neck split mystery

[PaulB47]

3 years ago I made some .300 AAC BLK for my Handi rifle from PMC .223 brass that I had shot in an AR-15. I tried a few loads, then set the batch aside. Couple of days ago I went to that batch and pulled the Speer 100 gr plinkers out of the 5 remaining loaded cases and loaded them with the Hornady 90gr XTP bullets over 5gr of Trail Boss. I shot them and got a neck split in all 5 cases (none of the other 15 that I shot earlier had split). I had not annealed.

The two shots I had chronoed and saved gave me 1062 and 1142 fps.

It was weird loading them. I would drop a round into the chamber with the bullet seated long, and then seat deeper and deeper until the case (barely) went all the way in. But I ended up with only 50 thousandths of bearing surface outside the case. The Plinkers were seated much farther out and still didn't hit the rifling. The OAL with the XTPs was only 1.606". That was as deep as I could seat them because my Wilson seater ran out of adjustment. The XTP is a .309 bullet rather than .308; I wonder if I was running into the throat rather than the rifling?

Like my other Trail Boss loads, the necks did not expand. Strange to see neck splits with low pressure loads.

I had another similar experience with a 6.5 TCU gun I have, that time with full power loads. No case neck splits at first, then after a few years I get mostly neck splits. Winchester brass, not PMC. Do cases just get brittle from sitting for a few years?

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[Member 34319]

Reading this correctly, you pulled a bullet and sat a .001 bigger bullet? That sounds like extra stress to the neck on brass that probably should of been annealed.

 

[PaulB47]

Possibly. It was not hard to seat though. Keep in mind many people seat lead bullets that are .311 or .312. Also, with this case, the neck is formed by squeezing down the body of the .223 case from something like .38 inch. Going back up a thousandth doesn't seem like it would be a big deal.

When people make .300 AAC cases, do they always anneal as part of the process? I will have to do some reading.

 

[Member 5605]

When people make .300 AAC cases, do they always anneal as part of the process? I will have to do some reading.

No, not everyone.
You have thicker brass where you trim, if you were not neck (thickness) trimming, you could easily get a split because thats a lot of brass to move. Even annealing isn't a sure fire way to avoid a split.

Some brass just isn't as good.

 

[PaulB47]

According to this:
http://www.massreloading.com/300BLK.html
... my PMC cases are marginal. However in the cases I fired full power loads in, before resizing, a .309 bullet slides in just fine, although not with much slop. So I am going to not worry about it, but stick with R-P cases from now on.

The info about annealing is annoying, though. I don't have $450 to buy a machine to do it. I have noticed that .300 AAC cases are very cheap so I may just buy a bag of them from one of the people who sells them (provided he anneals them).

I guess, all in all, .300 AAC are not so easy to make after all, due to the annealing issue. A shame...

 

[PaulB47]

You have thicker brass where you trim

As I understand it, the 300 AAC neck dimension takes into account the brass thickness. Although clearly not for every case brand! I suppose the good news is that with the right brass, you get a benchrest style tight neck situation, which helps accuracy (not a consideration with a semi-auto though).

 

[DizzyJ]

Brittle or low quality brass?

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[DizzyJ]

No, not everyone.
You have thicker brass where you trim, if you were not neck (thickness) trimming, you could easily get a split because thats a lot of brass to move. Even annealing isn't a sure fire way to avoid a split.

Some brass just isn't as good.

Was going to mention the converted brass (from .223/5.56 to 300blk) will have thicker case necks, and more work hardening than purchasing true 300blk brass.

 

[Hummer]

When I was working for the Army Small Cal Lab at Picatinny Arsenal there was another highpower shooter that was a ammo engineer who had come there when Frankford Arsenal closed. He worked about 1/2 mile from me.

We would talk at matches and he gave me a education on cases. I remember the quote like it was yesterday which was "You should never have a split neck if you take care of your cases and the only reason to lose a case should be from the primer pocket getting bigger.

He taught me how to anneal which actually meant stress relieve the necks. I don't remember getting a split neck on anything but 357 magnum brass in the last 30 years.

Now that being said it also helps if you have a tight chambers as most of them are big enough to park a blimp in. When I buy a chamber reamer it is speced to the 222 principle which means I don't want a case moving over .002" at the neck, should or base when it is fired.

I have 500 dedicated cases for one match rifle that are on their third barrel now, never lost one to split.

I have one LC 65 Match 30.06 case I have loaded 157 times and it is waiting for more.

 

[Grumpa]

Any time you stretch case mouths, you run the risk of this happening.