AR15 head space

[someguy2800]

Hey guys. I have been playing with my AR15 for the last couple months on and off trying to get it to group nicely without much luck. I have had 3 barrels on it, a 16" delton 5.56 1:9, a 20" bergara .223 wylde 1:9, and now a 16" Bear Creek 223 wylde 1:8 5R. I have not been able to get any of them to group better than 2 inches at 100.

The gun is a bear creek arsenal side charging upper, an anderson lower, A2 fixed stock, nikon M223 mount, bushnell 3-9x40, and a couple different free float tube.

I don't know why I didn't think of it sooner but tonight it finally dawned on me that I should measure the headspace. Using a rcbs mic to measure the shoulder of the brass my fired brass is measuring about .012" longer than a full length sized case. I checked 3 full length 223 dies I have on hand they all give the same sized length. I measured about 10 fired cases and they are all .012-14" longer to a datum point on the shoulder than a sized case. I grabbed some cases that were fired out of the delton barrel and they were .010-.012" longer than a sized case. Just by putting them next to a sized case you can visually see the cases are blown out quite a bit.

Now I'm used to a bolt gun only blowing the shoulders forward a couple thousands of an inch. Is this normal for an AR15 to blow the shoulder out this far or is the headspace excessive?

 

[brasscollector]

I haven't referenced any SAAMI specs but .014" sounds a bit excessive for 223. Of course if your tool has a radius on the edge that contacts your datum there could be more there than what you're reporting too.
Did you use the same bolt head during all of this barrel swapping?

 

[243winxb]

Not normal. Bolt problem if more then 1 barrel is producing fired brass that long.

Do buy a headspace gauge to check the chamber.
https://www.midwayusa.com/product/383456/forster-headspace-gauge-field-length-223-remington

At a difference of .014" this will produce case separations. May take more than 1 firing.

Good read - https://www.thehighroad.org/index.php?threads/ar15-new-build-and-possible-headspace-issue.824277/

Op' build- https://www.thehighroad.org/index.php?threads/my-first-cheapo-side-charging-ar15-rifle-build.825317/

 

[444]

I would say that is not normal.
.014" is WAY too much.

But, you are setting your shoulders back way too far when you size the case. Back off your sizing die until you size it about four thousands of shoulder setback. You need more than a bolt action to ensure reliable function but only a fraction of what you are sizing them to, now.

 

[243winxb]

my fired brass is measuring about .012" longer than a full length sized case.

Is the FL sized case being sized to much? My Dillon shell plate produced a different head to datum measurement from each station. The worst was .014" and produced case separations.

A single stage press, die and shell holder will produce about .005" shoulder set back. This is when comparing fired to FL sized.

223 autos may open the action to soon, if using a suppressor.

 

[Blue68f100]

Using the same bolt with the barrels will more than likely give you a different headspace. The only way to check is with Go-Ngo gauges. But on a simi-auto you are pulling the brass out under pressure so more stretching is going to happen than a bolt gun. But normally it should be in the 0.005" range not double that. Does the gun throws the brass forward? If so it's a indication that the bolt is moving too fast and needs to be slowed down. The brass should be thrown in the 4:00 direction. There are several ways to slow the bolt down. The best way is with a heavy BCG and buffer. Another way is the use of an adjustable gas block. Most go the adj gas block due to it's less expensive and gives you a larger tuning range. My AR's all have min spec chambers with adj gas blocks. My brass fired in these gun would fit a Wydle chamber, which is considerably larger without sizing.

Now you say your having trouble getting accuracy. I was never able to get my Wydle chamber AR to shoot anything leas than 65gr accurately. 55gr of any kind would only give a 1.5-2" @ 100 yrds. 69gr SMK would give me dime size groups.

 

[boom boom]

You really only need a no-go gauge because if you were having too short headspace, you would have had other problems than stretched brass.

 

[someguy2800]

Thanks for the input guys. I just wasn't sure if AR's tend to blow the shoulder forward as the unlock. I don't know why I didn't think of it but I load for a couple family members AR's and I have brass from one of them at the moment. I measured a case from that gun and it was .004" longer than sized. It sounds like the headspace is definitely excessive on my gun. I know my dies are in spec as I checked all three 223 dies I have and they all gave me the same sized length as a factory round I have on hand.

As for the gassing on the gun this is a 16" with a mid length gas port, brass always ejects at 4 o clock and just from sitting behind the gun the bolt carrier speed seams very mild compared to the other AR's I've shot.

As mentioned I could correct for it using the sizing dies but I do not want to do that because I want the gun to be able to fire any brass, not just brass sized for this gun. Since the problem carried across two barrels I think the problem is probably the machining of the bolt so I will get with the manufacturer to get it taken care of. I don't have go/no go gauges but I know someone that likely has them so I will see if I he will let me use them to check it out. I skipped that step when putting this together on the advice of somebody who told me he's never seen one out of spec. Lesson learned!

 

[someguy2800]

Now you say your having trouble getting accuracy. I was never able to get my Wydle chamber AR to shoot anything leas than 65gr accurately. 55gr of any kind would only give a 1.5-2" @ 100 yrds. 69gr SMK would give me dime size groups.

Thank you for that advice. All the bullets I've tried were 50-62 grains with the only acceptable groups coming from the 60-62. I originally got a 1:9 barrel wanting to shoot 50-55 grain varmint bullets, but I was advised by someone local that a 16" barrel length would do better with a 1:8. This was contrary to my own ideas about 223 twist rates but since I had already failed twice with 1:9's I thought I would give it a try. My next step was going to be to order some 69's and see how they do, but I'll have to fix the headspace before I waste any more powder through it

 

[IMtheNRA]

What is the actual load that you're using? Bullet brand, weight, powder and OAL... You should be able to get better than 2" even with a mismatched bolt and barrel.

If you're using some military bulk bullets or pulled used bullets, then 2" is really good.

 

[someguy2800]

The list of loads shot would be too long to write. Between the two barrels, 8 bullets, 3 powders, 3 primers and lots of combinations and charge weights. It flat out won't shoot. All quality components, most of which shot 1/2 moa in my savage.

 

[dirtman]

Hi someguy, I have used my rcbs mic to gauge the set back when i bump back the shoulder .002" for my AR and it has worked great... then i test that all case's will cycle thru the gun and load em up.... This way you are not over working the brass and they pressure seal all about the same when fired.. The only down fall is the brass has been sized for this gun only..... I keep my partial sized brass separate for each gun.
good luck.. and have fun...

 

[Toprudder]

I have used the Redding +0.006" competition shell holder for resizing my 223 brass. I've checked that they will chamber in all my ARs with a little room to spare.