Questions about neck thickness in 300 Blackout

[mtgrs737]

Now that I have my 300 BO upper I am looking at making my own cases from 5.56 x 45 military brass and have some questions about neck thickness. I would like to say that I did buy some cases from Grumpa here on the board and they are excellent cases, but being a do-it-yourselfer I want to be able to make my own. Looking at the SAAMI standards for both the chamber and the cartridge it looks like I have a maximum of .013" neck wall thickness if I were using a .308" dia. bullet. If the chamber is cut to the largest size the .013" neck wall thickness would leave no room for the case to expand and release the bullet so I would like a .001" or so clearance in this area. However I am planning on using cast boolits in this rifle and will probably size to .310" or .311" dia. which if I have the maximum neck wall thickness will cause a problem with safely chambering. Have any of you guys addressed this issue and if so what did you do to reduce the wall thickness of the neck area? I am wanting to make sure that my brass is safe to shoot with oversized cast boolits.

Thanks!

mtgrs737

[destrux]

You shouldn't need to run .311 diameter, mine runs cast just fine at .310". Generally the chambers are loose enough dimensions that even .311" with no crimp will fit in the chamber with no problems though if you find you need .311" to seal things up. (edited because I forgot what sizes I was talking about)

There is a list of brass that is suitable for conversion to blackout at this link... http://www.300blktalk.com/forum/view...?f=141&t=88599

Not all .223 and 5.56 brass is usable for conversion, but I have run batches of misc range brass without yet finding a case that was too thick in the neck.

[Jupiter7]

Foreign made 5.56 brass is the most common offender regarding neck thickness. I've purchased brass and formed my own and shoot cast exclusively. My normal cast size is .311" and neck wall thickness hasn't been an issue.

Load some ammo and chamber check it. If it chambers freely, it's fine.

Mirrors my experience. I also run .311 in all mine.

[mtgrs737]

Thanks guys, that really helps me to think I should not have any problems with neck thickness. I have not and probably will not make a chamber cast to see just what I have, but will stick with LC brass for my conversions and I will check the neck thickness. I plan on only shooting cast in this cartridge so I may hit you guys up for some data that is working for you.

[wlc]

Well, I've never thought about it honestly. I made some of my own and decided that buying from Grumpa was a smarter move, for me, at least. I shoot only cast now as jacketed bullets are harder to find in my neck of the woods (haven't seen 208gr AMAX in over a year) and too expensive just to punch paper with. That said, I size to .309, have no leading problems, and my loads cycle, BHO and are accurate at 50yds. I may try sizing and loading a couple of dummy rounds at .311 and see how they chamber. My home made brass works well too, it is just waay more time consuming to make. I'd rather have teeth pulled.... I made my home converted from once fired LC brass and Grumpas is as well.

[Ben]

As to shooting .311" cast bullets, - - - take a fired .30 cal. case that comes from the rifle that you're loading for.

Take a sized .30 cal. cast bullet ( .311" ) , if it will slide into the neck of the fired case with minimal or no resistance, you're good to go.

[sundog]

mtgrs737, seems to me the easiest way to reduce neck wall thickness is outside neck turning. Not only reduces thickness, but also uniforms the thickness. I outside neck turn a lot of brass, and most of what I see is neck wall thickness is not very uniform on most cases - that is, until it's turned. It is labor and time intensive, but in my estimation it improvement worth the effort.

[popper]

No guarantee the chamber/bore/groove is anywhere near SAMMI standards. Ben's post is pretty spot on, for a just fired NEW super sonic case. Then measure/record the OD of the fired case. That is close to your max neck ID. Size & expand your formed cases, check OD, add 0.001". If greater than recorded number, probably have to turn or toss. Load ammo, check against the recorded size - pull those that won't fit. Bolt closing will NOT resize the case & pogo-ing to remove a stuck loaded round is not fun. I've actually had pretty good luck pulling PCd cast with the Honady clamp puller.

[Moonie]

I seem to be the oddball out, If I don't neck turn my brass and size to .311 I have about 1 in 10 hang up in my chamber. My sons upper does not and will feed and fire those that hang in mine.

I figured my chamber was just a tad bit tighter than his.

[petroid]

^^^Check the list, it saves a lot of hassle. I pretty much use LC or other domestic brass only for 300BLK conversion. No problems at all sizing to .310 which is what my Lee .309 sizer puts out. Pound cast of chamber indicates it should fit a .311 boolit but just barely.

[popper]

The list is a general guide only. I've got cases from the bad list that work fine, ones from the good list that won't. Even worse, I've got cases fired in the same chamber/boolit that are too large now. Don't know why. I annealed (candle method) a bunch of brass to see if it makes a difference. Grain size changes a lot with annealing.

[truckjohn]

Remember that saami spec for chambers only list minimum dimensions. That spec lists the smallest allowable chamber for that specific cartridge.

Generally - especially in self-loaders - they never cut chambers to minimum spec due to loading and ejecting concerns.

That being said - shoot a couple rounds of commercial ammo and mic the neck od and id of fired rounds to see how much room you really have....

[kweidner]

Maybe some of us are over thinking this? I have 3 blackouts, my cousin 2. granted I size to .3095 but haven't had one not chamber yet. All 5 guns mentioned shoot 1.5 MOA at 100 cast and hytek subsonic. 2 will shoot sub MOA. I do honestly prefer Gorilla brass for my hunting stuff as it is super consistent but for plinking AR fun I will use range brass of any make. I use a harbor freight mini and a little crow for trimming. I can cut, size, take mil crimp out and trim/ debur 100 in about an hour. I literally did close to 1000 just last week. Played this weekend with my swaged design and was shooting MOA at 250 yards suppressed with my encore. Pffft. wait.... wait.....wait....DING. really brings a smile to your face. I will take any brass that ain't crushed. Send em to me. I'll convert em and shoot em��

[mtgrs737]

Thanks again guys for all the replies, when is comes to simi-autos I am more concerned with chamber fit and functioning and of course pressures than with bolt guns due to not being able to feel the chambering.

[Jeff Maney]

I have formed about 4K pieces of 300 BLK using LC05/09 ball and LC06/10 blank brass, no problems with any!

Most neck wall thickness has been about 0.011 to 0.012" with all LC brass. I personally like using the LC blank brass

since the case has not been really expanded due to the light blank load. Almost no effort to size/form to 300!


Jeff

[maxiblu]

I recently had chambering issues with .309 Lee 160 gr. I had made my cases from mostly once fired military, but some were commercial cases. After much experimentation and measuring, I found SOME but not all, PPU cases and MKV 14 cases measured necks that were .004-.005 larger than other cases after full length sizing. Remember when sizing, the case mouth goes over the expander, than neck gets downsized in the die, but than the mouth comes back over the expander when exiting the die. So, some of the above mentioned cases had thicker walls than others, leaving a larger diameter neck that would jam before completely chambering. I apparently have a tight chamber in my AR barrel. Out of approximately 400 cases I had 12 with thicker brass in the neck. My solution is outside neck turning since that will lead to more uniform cases anyway. If you don't want the hassle, just throw any oversize into the scrap brass bin.

Just relating my experience on the subject.

By the way, these oversize cases worked ok with jacketed bullets. .002 does make a difference so size does matter after all....