7.5" AR pressure conundrum

LUTNIT

LUTNIT

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So I have an NEA-15 with a 7.5" barrel I'm trying to get to work that so far has been problematic.

Factory Federal 55gr 5.56 ammo with Lake City headstamps runs flawlessly and has an absolutely massive muzzle flash. I get flame shooting 10" out either side of the 4-port muzzle break. Extracts, ejects, and cycles perfectly.



I tried reloads with Campro 55gr bullets and RP brass using Benchmark powder. Load data shows 24.0-25.6gr for that weight with a cup-and-core bullets.

I tried 24.0, 25.0, and 26.0gr loads and they all caused cases to stick horribly in the chamber so badly the extractor was ripping the rims off. Even with the bolt open and a cleaning rod I needed to tap the cases out with some force. (The 26.0 was existing ammo I had for a previous AR that worked well so I tried it since I had some lying around; I don't normally go above book max).
I went under to 22.0 and 23.0gr loads and to my amazement I had the exact same problem. Stuck cases in the chamber and ripped off case rims.

One big difference is none of the Benchmark loads produced much of a muzzle flash. Not just smaller than the Federal 5.56 factory ammo but nearly non-existent.




The gas port is much closer to the chamber so there would be much higher pressure there than on a carbine or longer gas system. So the bolt may be trying to extract the cases before they've had a chance to spring back and release from the chamber walls. I put a CNA adjustable gas block on it and turned it down to where it barely cycles the action and I still get a case stuck every 2-3 rounds fired. Again with ripped off rims. The rims alone I could possibly explain with the shorter gas system but that shouldn't cause the cases to stick like they are. One guess is perhaps Benchmark is too fast of a powder for this gas system (based off the muzzle flash)? But why would cases be sticking with loads 2.0gr below start loads?

It's the fact that the warm-ish Federal 5.56mm loads run flawlessly that is really screwing me up. They aren't reduced or low-pressure loads so if it was a pressure or gas system issue, they should also cause problems, but they don't. That's why I thought of maybe the powder burn rate but that doesn't make much sense for the sticking cases either.
 
Load data from hodgdon show the same load so it should be good.

Did you mix up powder? Scale giving bad reading so more/less powder?
Maybe take some measurement against a factory load (COAL, datum, etc)
Measure the bullet?
 
From another forum:

Looks like a round that failed to fully extract when fired, and then the casing got rammed back into the chamber without extracting.

A fired case, when rammed back into the chamber will often "sieze" in the chamber, requiring excessive force to remove it.
If the "excessive force" is not applied correctly, the "forcer" (that's you bubba) will often rip the extractor over the case rim, and causing EXACTLY the sort of damage we can see in this case.
 
Hmmm.... so could be a weak extractor. I wonder if the different thrust from the reduced loads and factory loads causes the issue. More testing is needed. And maybe invest in an enhanced extractor spring.

The same batch if test ammo ran without issue in another AR so I don't think the scale is off. The light loads didn't lock the bolt back on the second rifle either.
 
Fails to extract and then slams the already expanded case back in the chamber back into the chamber? Sounds like you are under-gassed maybe. Benchmark might not be a good powder for that application. Maybe the Lake City ammo works because it using a slower powder?
 
You have high port pressure and softer Remington brass and you will not like what I'm about to say.

Size the Remington cases with a small base die, a resized case in a semi-auto should be .003 to .005 smaller in diameter than its fired diameter. This allows the case to spring back from the chamber walls and extract reliably.

Or resize the Lake City brass you have with your die. and with the same loads as the sticking Remington cases and see if they extract.

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How Hard is Your Brass? 5.56 and .223 Rem Base Hardness Tests
http://bulletin.accurateshooter.com/2014/05/how-hard-is-your-brass-5-56-and-223-rem-base-hardness-tests/

brasstest03.png
 
Ganderite, I have the rubber spacer in the spring but no o-ring. Someone I talked to in person mentioned the o-ring as well. Maybe I need to try one.

Jethunter, I'm not sure if it's extracting at all or just skipping over the rim from the start. I have some loads with IMR4064 I was going to try this weekend. Maybe a slower powder will make a difference.

Bigedp51, I had thought of that as well since the Remington .223 brass is soft as butter. I have a bunch of LC and IVI military 5.56 brass I was going to try as well. Make up some loads with Benchmark, H4895, and Varget and see if there's any difference with different burn rates with the harder brass.
 
LUTNIT said:
Ganderite, I have the rubber spacer in the spring but no o-ring. Someone I talked to in person mentioned the o-ring as well. Maybe I need to try one.

Jethunter, I'm not sure if it's extracting at all or just skipping over the rim from the start. I have some loads with IMR4064 I was going to try this weekend. Maybe a slower powder will make a difference.

Bigedp51, I had thought of that as well since the Remington .223 brass is soft as butter. I have a bunch of LC and IVI military 5.56 brass I was going to try as well. Make up some loads with Benchmark, H4895, and Varget and see if there's any difference with different burn rates with the harder brass.
Click to expand...

That will increase port pressure. I am guessing a faster powder would be the way to go.

When you load 4064, load from 15 gr to max and see what happens. If the lighter loads work, you will know the problem relates to high port pressure.
 
Would a slower powder increase port pressure in a pistol length gas system? I've always been told a .223/5.56 needs about 10" of barrel to burn all the powder, so if the gas is being siphoned off only about 6" from the chamber, would the powder not all be burned yet? Would a faster powder burn more completely by that point (thus generating more pressure)? I'm no expert on these things.

Benchmark is already pretty fast. I have some H4198 I could try as well to see if that makes a difference which I think is a touch faster.
 
Tried several different powders in IVI brass. All have the same issue. The Federal 5.56 LC ammo is now also causing the same issue so must have been a fluke before.

Mostly failure to extract. Lots of bent or gouged rims by the extractor. About 1 in 3 rounds. From below start to max loads of H4198, H4895, IMR4895, Benchmark, Varget, and IMR4064.

Had a single failure to eject where it tried to put the empty case and a live round in the chamber at the same time. That was a light load as well. Only happened once.

My current guess is the bolt is being pushed back when the chamber pressure is still high or the brass hasn't had time contract so the case is still stuck in the chamber. Extractor bends or gouges the rim while it pulls back.

So if I need to slow down the gas impulse that's pushing the bolt back I guess I could go with a heavier buffer, stronger buffer spring, or longer gas tube? The NEA CCS stock has a buffer built into the bolt carrier which only takes 2 weights instead of 3 in a normal buffer. The buffer spring is also a proprietary design so I can't replace it. Maybe a "pigtail" gas tube would help? Dlask sells individual tungsten buffer weights I could try in the bolt carrier.

I didn't have my other AR out which I intended to but forgot so I couldn't swap lowers to see if a conventional buffer setup would help.
 
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