300 AAC Blackout Case Head Sizing Problem

[Liberty1776]

After resizing my .300 AAC Blackout cases using an RCBS Small Base (SB) die, then trimming to length (if necessary) and deburring, most cases fit into my Hornady 300 AAC case gauge

But about 1/4 of the lot will not "plunk" into the case gauge, and stand proud about half the thickness of the rim:



The rims are simply too large to fit the Hornady gauge.

This shows how tightly the rim goes into the gauge:


This photo shows how a "plunker's" rim easily fits into the gauge:


To add confusion, a Dillon .223 gauge is several thousandths wider at the rim mouth of the gauge than the Hornady, though both .223 and .300 AAC cases are theoretically the same dimensions.

A tight .300 AAC case that hangs in the Hornady gauge falls easily into the .223 Dillon gauge.

Should I buy a different gauge?

Can a .300 AAC case be "debulged?"

What's your go-to 300 AAC Blackout case gauge, or am I overthinking this?

Thanks.

[BK7saum]

I don't and never plan to use a case guage. I check/set up my dies and size/measure the first few cases. Once I have the dies set to size brass for my chamber, I then proceed to process the batch.

I don't need or want my brass to be overworked and there is no need to have my brass fit every chamber out there. That can negatively affect
accuracy of my ammo.

My opinion, you are overthinking it. My suggestion to anyone is avoid using a case gauge and set up your dies/brass for your rifle. If you have multiple of the same caliber and need to use ammo in both, set up for the tighter chamber and you're good to go.

To each their own. I find a case guage unnecessary. As you found out, the rim is being measured/rejected when it is irrevelant to fit in the chamber.

[BK7saum]

You are seeing a false indication of incorrect headspace due to the rim width or having burrs. Clearly the rim/case head will still fit in the bolt recess. The case guage may meet some spec in this area but could be opened up slighlty at the head/rim area and still give the appropiate headspace indication.

But that gets the brass back to SAAMI spec or other measure that doesn't relate directly to YOUR chamber.

[Liberty1776]

I employ a case gauge as a final inspection on my ammo, especially for 9mm, .38 Special and .357 Mag.

I've caught high primers and especially split cases, which seem to split when the bullet is seated. I find the split because the finished cartridge doesn't fall into the gauge right -- something is hanging it up.

A case gauge also tells me if I'm setting the shoulder correctly. Early on, when I didn't know that much about necked cartridges, I screwed up reloading .308 Win and pushed the shoulders back, causing a raft of case head separations and difficulty to extract from my 700 Remington.

I do have two different 300 AAC guns, two different .308 Win rifles and multiple .223's. Don't have the luxury to load for only one chamber.

[Liberty1776]

You are seeing a false indication of incorrect headspace due to the rim width or having burrs. ...

Yes, exactly. Every case fits except for that last few thousandths. Very annoying. And it's really hard to do a chamber fit check on an AR platform, seeing as the cartridge is WAY down there in the star chamber.

I checked for burrs on the rim, thinking the extractor or ejector damaged the rim, but they seem fine.

I am tempted, as you say, to polish open the Hornady's big end to let brass fit.

As a side note, factory SIG 300 AAC nickel plated cases drop in perfectly every time. Lake City converted 5.56 brass seem to have the most problems.

[imashooter2]

Don’t have one, never needed one. Does the bolt close on the case the gauge says is no good?

[dannyd]

When I make 300 blackout brass from 223 or 5.56 I turn the neck down a couple of thousands and it fits the case gauge fine. I use a case gauge for all the rounds I make.

[ReloaderFred]

I've never fired a round out of a case gauge, yet. My chambers are my case gauges. Once my dies are set up for my chambers, I'm good to go.

I do have a complete set of case gauges for the common handgun rounds, but I can't remember the last time I took them off the shelf to check a round.

Hope this helps.

Fred

[farmerjim]

If you have a neck turner tool it can be adapted to turn down the rim using a mandrel that fits the primer pocket.
I just made 1,000 300 BO from 223 and only about 50 had too big of a rim.

[Liberty1776]

If you have a neck turner tool it can be adapted to turn down the rim using a mandrel that fits the primer pocket.
I just made 1,000 300 BO from 223 and only about 50 had too big of a rim.

Have never turned down a case neck. Interesting that you turn down the rims. Both the 300 AAC and the 5.56 NATO spec a .378" rim diameter. No idea why that rim would change or need turning.

[jmorris]

I don’t own a case gauge for 300 blk but I also don’t shoot it in competition or have tons of them either.

Assuming you are loading for a semiauto, this works just fine.

[jmorris]

If you are loading for a given bolt action, I have the best results using this method.

[megasupermagnum]

Hornady is a good company. I would be willing to bet that if you explain that your case rims fit in a 223 die, but not the 300 BO die, that they will send you a new 300 BO die. I don't think there is anything wrong with your rim width. They get dings and such from being fired and extracted, but it's not a critical dimension.

[Liberty1776]

Well, a solution is right there in my initial photographs, where I reversed the case and put the rim end in first, proving some rims simply don't fit.

I needed to sort my .223/5.56 brass using the Hornady Case Gauge's large end. Why fight the brass? I've got thousands of spent cases I've picked up at the range -- just use the ones that fit.

So I fit check the brass before I begin the conversion process to .300 BLK.

If the .223 case fits into the big end of the 300 BLK case gauge by 1/4" or more, it will size and "plunk" after I work it.

If the rim is too large to fit into the 300 BLK case gauge, or just goes in slightly, I throw it in the pile for reloading into .223/5.56.

If it's a converted .223 round that doesn't fit, retire it. Why fight it?

I have many, many cases to choose from, and the Hornady case gauge works nicely as the sorting tool.

[farmerjim]

Neck turning puts the bullet in the center of the neck for improved accuracy.
In the case of 300 BLK made from 223, the neck of the 300 BLK is made from the main body of the case. In some cases the brass thickness plus the 224 bullet is greater than the size of the throat. This can cause excess pressures.
There is a list of the brass that is good and bad for conversion.
You can measure the thickness with a tube micrometer, or if you have a neck turning tool you can set it to turn down any that are too thick.
Most of the brass I converted were fine ( not too thick) only about 30 were too thick. However, most of the necks were out of round. Not a big thing unless you are competition shooting, but the neck turner is faster than the micrometer.

[popper]

Rim dia is not that important if it fires and ejects. Rim dings can cause the next rnd in the mag to get pulled forward and jam. If your sizer die is properly set when you run a batch, all the shoulders should be in the correct location, fitting in gauge or not. The gauge is good to check H.S. on the start of a batch. I use a wilson.

[oley55]

PIC two shows the rim sitting below the surface of the gauge. So it's the body of the case that is making it proud.

[jmorris]

This machine is designed to size the portion of the case a normal die can not.

http://www.casepro100.com/

[Liberty1776]

This machine is designed to size the portion of the case a normal die can not.

http://www.casepro100.com/

I love the idea of roll sizing back to factory spec.

I don't love the $900 price tag....

[Milky Duck]

. Lake City converted 5.56 brass seem to have the most problems.

well there is probably your problem.
I do not shoot the 300AAC BUT DO SHOOT .223REM and have done so for years using a sack of milserp brass I was given...if I sort headstamps there is 4 different types,one of which has always given me trouble so it gets chucked out..I BELIEVE this was possibly a hotter tracer or heavier load ,powder charge ,projectile weight or both.... so the case got a jolly good whoomph on its one use and hasnt shrunk back as small as it should.... if you think about it,how can that rim be wider unless it was to begin with,EG was manufactured bigger to begin with...the only possible way it,the rim.could grow outwards would be brass flow....and when do you get brass flowing that way,that much????? the issue I had was case jamming in LEE auto prime,oh man did that do my head in,remove tray of primers,making sure to clear the on ramp,try forcing primer in deeper and then try to get case out,some would stick in shell holder on press too,it was like the rim itself was out of shape/off square or too thick.
in the .308 the only brass we have ever had issues with was S&B it was just too hard and felt like press handle was going to tear off.
chuck a micrometer on a not fitting case and see whats going on....
but you over thinking it,if it fits in rifle n fires..where is the problem again????