5.56 NATO Marked Barrel Pressure Problems

[GuitarmanNick]

Today, I did a couple of ladder tests on 2 different guns, both with 5.56 NATO marked barrels. I ran the same tests on both guns and began with the 16" because the scope is much better. In the next to last test, it popped a hole in the primer on one round out of five. The last test, it popped holes in two primers. The last three tests all showed some signs of building pressure.
Test loads were all well below the max .223 load in Lyman's 49th. All brass was once fired, matching headstamp and trimmed to the same length. Primers went in normally and to the correct depth. I was quite anal about today's loads and wanted to rule out my brass as a contributing factor.
A little background on this build: Purchased the upper assembled and complete with BCG. Tested with a 5.56 field gauge. Passed. Mounted a scope and started developing loads for it.
It has shown pressure signs much earlier than expected on every test run thus far. I never get anywhere near the listed max load for any powder before primers get popped or blown out entirely.
Next, I tested the same ladder on the 20" gun without any pressure signs throughout. This gave me a good comparison between the two guns with the exact same loads. Noticeably different primers!
My thinking is that the 16" has a very tight chamber and may actually be tight for a .223 Remington.
I have not run any 5.56 ammo through it yet, and do not think it would behave well based upon my testing.
I have a spare BCG I could try, but do not see that being related to the gun's issue.
I would like to know it is safe to shoot 5.56 ammo in it. Realizing that it would have crimped primers, perhaps I am overthinking the issue.
Suggestions and/or ideas are welcomed.

 

[outrider58]

You'd be surprised how many 5.56 nato chambers test out to have more of a .223 lead.

Have you checked the firing pin protrusion on the 16" gun yet?

What primers are you using?

 

[GuitarmanNick]

Firing pin protrusion is .033". Near the max, but within specs. Tip looks good, too.
Primers are CCI 400 SRP.
Poor quality control seems to be SOP these days so, not much surprises me anymore.

 

[Rocinante]

Is it a pierced primer or a blown primer?
What is the gas system on the 16"?

 

[outrider58]

Firing pin protrusion is .033". Near the max, but within specs. Tip looks good, too.
Primers are CCI 400 SRP.
Poor quality control seems to be SOP these days so, not much surprises me anymore.

You should be good out to .036"

400s are pretty stout primers. I doubt they were the culprit.

 

[85MikeTPI]

You should be good out to .036"

400s are pretty stout primers. I doubt they were the culprit.

450s have thicker cups for CIP loads and floating firing pins

 

[303_enfield]

With the firing pin protrusion near max and running softer cup primers that's what you get (with floating firing pins). Have any CCI # 41 primers on hand? A tighter spare bolt group?

NATO marked but what twist? Bullet weight?
Did you run the ammo over a coronagraph?

Cast the chamber to rule that out also.

 

[John from MD]

I suggest doing a chamber cast to see if you really have a 5.56 chamber.

I have shot tens of thousands of CCI400 and they are no problem in the AR platform.

 

[outrider58]

450s have thicker cups for CIP loads and floating firing pins

True, but #400s are tough too. I mostly use WSRs which are 'notoriously' soft and have never slam-fired nor perforated one. I've only had one blown out primer. True, I don't load to NATO pressures, but in this case, GMN didn't either.

 

[GuitarmanNick]

Is it a pierced primer or a blown primer?
What is the gas system on the 16"?

I have experienced both with this build. Started blowing primers on a test once about halfway between the suggested starting charge and max for .223 Remington.
I am not trying to load to 5.56 specs. Just want the gun to be capable of using it without problems.
Carbine gas system on the 16, rifle on the 20.

 

[Rocinante]

I have experienced both with this build. Started blowing orimers on a test once about halfway between the suggested starting charge and max for .223 Remington.
I am not trying to load to 5.56 specs. Just want the gun to be capable of using it without problems.
Carbine gas system on the 16, rifle on the 20.

Sounds to me like your problem is the 16" is stupidly overgassed and has horribly short dwell time, and not your reloading skills. A 16in carbine gas port size should be between 0.062-0.07".

 

[Park ranger]

I won't use cci 400 in a AR with 5.56 pressure

 

[KRC]

Just an anecdote, possibly related . . . ?
My 6BR, built on an aging Hart 1A action (~50+y/o with a Rem M700 bolt) began blowing primers at loads it had not previously. These were not hot loads (i.e. 29.6 gr. Varget with 105-108 gr. bullets). Primer pieces were being blown into the bolt. The problem turned out to be excessive wear on the firing pin hole giving too much clearance around the firing pin. Sent the action to Mike Bryant who drilled out and "bushed" the firing pin hole to a smaller diameter. Problem solved, and the rifle is now capable of shooting very hot loads. (Which I don't.)
Check firing pin hole diameter?

Also, check barrel bore diameter if possible. Excessive pressure could be due to an out-of-spec tight bore. (Using the same very tight patch setup for both, is there a difference in resistance feel when running patches through the two barrels side by side?)

Also, can you borescope the leade and throat just to make sure something is not way-wonky with the chambering job?

 

[GuitarmanNick]

Sounds to me like your problem is the 16" is stupidly overgassed and has horribly short dwell time, and not your reloading skills. A 16in carbine gas port size should be between 0.062-0.07".

The gun cycles fine. Overgassing would not cause blown or popped primers.
It would more likely cause cycling problems and additional wear.
After determining that this upper came from BCA, and some specific searching in a few forums, I am leaning towards this being a poorly chambered barrel.
I was able to find quite a few posts describing almost identical pressure problems with BCA barrels. Complaints about barrels did not discriminate between calibers, on whether sold individually or in an assembled upper. Some had returned barrels and the replacements were no better. Looks like a fairly common problem.
It was less than $200 with the BCG and charging handle. Guess I got what I paid for.
Don't think I will waste time with returning it. Replacing the barrel is easy and I am happy with the rest of the upper. It will allow me to select a quality barrel for the one AR that was hoping to use for more precise shooting.

 

[omegared24]

The gun cycles fine. Overgassing would not cause blown or popped primers.
It would more likely cause cycling problems and additional wear.
After determining that this upper came from BCA, and some specific searching in a few forums, I am leaning towards this being a poorly chambered barrel.
I was able to find quite a few posts describing almost identical pressure problems with BCA barrels. Complaints about barrels did not discriminate between calibers, on whether sold individually or in an assembled upper. Some had returned barrels and the replacements were no better. Looks like a fairly common problem.
It was less than $200 with the BCG and charging handle. Guess I got what I paid for.
Don't think I will waste time with returning it. Replacing the barrel is easy and I am happy with the rest of the upper. It will allow me to select a quality barrel for the one AR that was hoping to use for more precise shooting.

It's a BCA. What were you expecting and why would you invest that much time on it?

 

[outrider58]

It's a BCA. What were you expecting and why would you invest that much time on it?

BCA uppers belong on Anderson lowers... then tossed overboard.

 

[GuitarmanNick]

It's a BCA. What were you expecting and why would you invest that much time on it?

I admit to being aware of their poor reputation, and was hoping things had improved after they were raided for employing illegals. Not sure of my exact thoughts, but likely thinking that if it was a poor shooter, the barrel could be changed and the rest of the upper was worth tha price
I did read some very good reviews at the time of purchase, which gave me hope. Not the first time I have been disappointed.

 

[omegared24]

I admit to being aware of their poor reputation, and was hoping things had improved after they were raided for employing illegals. Not sure of my exact thoughts, but likely thinking that if it was a poor shooter, the barrel could be changed and the rest of the upper was worth tha price
I did read some very good reviews at the time of purchase, which gave me hope. Not the first time I have been disappointed.

I hear you. Unfortunately they meet all expectations.

I have an upper and it's taken some work to get it to run well.

 

[omegared24]

BCA uppers belong on Anderson lowers... then tossed overboard.

They really are a waste of money.

 

[erwos]

BCA barrel = all bets are off with your chamber spec.