Hornady 300 blackout overpressure issues

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mmcintyre1220

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Took my new 300 blackout build out today and ran into some issue I believe are strictly ammo based but I am looking for some input. The gun ran fine, cycled and locked the bolt back as expected but when I looked at my brass I was quite alarmed. I Beleive it was so overpressure it caused the bolt to unlock early making a belted 300 blackout 20180426_175905.jpg

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This is factory 110 grain vmax.

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Last edited:
Have you shot this build before or was this the first time? Have you tried any different ammo? Then again for now I don't think it would be wise to shoot it again until you get some ideas what may be contributing to that.

We need some details on your build. Barrel length? Gas system length? Standard or lightweight BCG? Anything else you can tell may help. It seems like the system is running way over gassed.

I'm having a hard time believing it's the ammo.
 
First time running this build. I have not shot another brand of ammo as this is all I could find at the moment.

Build is the following :

NEA 12.5 barrel with pistol length gas system and NEA gas block
Standard BCG
Spikes st2 buffer
standard weight spring

Rifle has been checked for headspace and it all checked out good.

I found a few other reported issues with this ammo as well as other pictures of what happens with very overpowered ammo in the at platform. It was also 300 blackout. The problem in that case was bullet setback with reloads. I have verified there is no setback with this ammo.
 
Kinda bizarre as it seems like the rifle is cycling OK. I wouldn't rule out the possibility of it being the ammo but I'm not so sure that a different ammo will make enough of a difference to eliminate what is happening. Seems pretty extreme to simply be an ammo issue. Not to mention you wouldn't get me firing different ammo in that with it beside my face.

I've never been a fan of the pistol length gas system. It puts a lot of gas pressure almost instantaneously into the bcg and the amount of pressure only increases as you increase barrel length past the gas port. One simple way to help reduce the gas pressure (or even it out) is with a pigtail style gas tube. It increases the length of the gas tube compared to a straight tube on the same length of system. In other words it allows the rifle to act as if it had a slightly longer gas system.

https://truenortharms.com/products/gas-tube-for-ar-15-choose-a-length?variant=31731261969

Or possibly an adjustable gas block to slow it down.

I've never ran this specific ammo so maybe someone who has will chime in here and let you know if they've had any issues with it before you start changing anything on your system.

Also, even though headspace was confirmed to be ok, does a round chamber without issue? Have you chambered a round and then manually ejected it without firing and inspected for any abnormal signs? Is it possible the barrel is throated too short and the bullet is contacting or being pushed back into the casing ever do slightly? This is a possibility that would drastically increase pressure and could potentially cause your issue. Measure the overall length of a new round and then chamber and eject it (without firing it of course) then measure the OAL again. If it is still the same then this is not an issue.

Trouble shooting can sometimes be as simple as trying different ammo and sometimes it's a little trickier to figure it out.
 
Do not fire any more of this ammo
Send a email or phone Hornady with Lot # from the box

Have you checked the bolt face ?

pressure is above what I would call normal for Saami for 300 aac blackout
 
jiffx2781 said:
Kinda bizarre as it seems like the rifle is cycling OK. I wouldn't rule out the possibility of it being the ammo but I'm not so sure that a different ammo will make enough of a difference to eliminate what is happening. Seems pretty extreme to simply be an ammo issue. Not to mention you wouldn't get me firing different ammo in that with it beside my face.

I've never been a fan of the pistol length gas system. It puts a lot of gas pressure almost instantaneously into the bcg and the amount of pressure only increases as you increase barrel length past the gas port. One simple way to help reduce the gas pressure (or even it out) is with a pigtail style gas tube. It increases the length of the gas tube compared to a straight tube on the same length of system. In other words it allows the rifle to act as if it had a slightly longer gas system.

https://truenortharms.com/products/gas-tube-for-ar-15-choose-a-length?variant=31731261969

Or possibly an adjustable gas block to slow it down.

I've never ran this specific ammo so maybe someone who has will chime in here and let you know if they've had any issues with it before you start changing anything on your system.

Also, even though headspace was confirmed to be ok, does a round chamber without issue? Have you chambered a round and then manually ejected it without firing and inspected for any abnormal signs? Is it possible the barrel is throated too short and the bullet is contacting or being pushed back into the casing ever do slightly? This is a possibility that would drastically increase pressure and could potentially cause your issue. Measure the overall length of a new round and then chamber and eject it (without firing it of course) then measure the OAL again. If it is still the same then this is not an issue.

Trouble shooting can sometimes be as simple as trying different ammo and sometimes it's a little trickier to figure it out.
Click to expand...

Jiffx2781 - Thanks for the suggestions ! The thing that made me think ammo is that some of the fired casings look fine.... well semi fine, they do still show minor pressure signs. Then there is 3 of them, 2 specifically that show major pressure signs and had gas escape from around the primer. The signs lead me to believe that it could not possibly be headspace or gas system related as I should have seen the same indications on all fired rounds. This round is right at the top of SAAMI spec and being hornady , like all hornady in my experiences has been hot.

Pistol length gas systems have never been my cup of tea. My only other experience with one was also negative but nothing to this extreme. I chose to go with pistol length in this build to have the ability to shoot heave 200+ grain sub sonic while maintaining full function of the rifle. Being NEA (again something I don't normally do) it wouldn't surprise me if the gas port specs are a bit off. I am going to order an adjustable gas block tonight to help rule out one issue. I have also sourced some 147 grain S&B ammo and Remington 220 grain which I will be trying out next weekend. I will be going heavy on the PPE and ensuring no one is beside me.

I have not measured the OAL but have cycled a few rounds manually without issues. Last night I separated the upper and lower and put a round in the chamber and closed the bolt by hand. The bolt closed and locked up perfectly with normal pressure. I inspected the round after and there is not a mark on it. I will take your suggestion and cycle a few and take measurements. It is it being pushed back it would be incredibly small which would shock me to be the cause of this.

I have had to trouble shoot a few issues over the course of multiple builds but this one takes the cake!

Thanks again for the help!
 
just having some fun said:
Do not fire any more of this ammo
Send a email or phone Hornady with Lot # from the box

Have you checked the bolt face ?

pressure is above what I would call normal for Saami for 300 aac blackout
Click to expand...

I will be sending some pics to hornady today. The bolt face had a nice ring around it from the gas escaping the primer but it did not cut it, it scrubbed right off. I took the bolt apart and everything looks to be normal.

I found a few reported issues with this round , including reports from hornady when others had sent ammo back. Hornady has reported soft brass as well as rounds loaded to "maximum" levels in the past with this 110 grain 300 blackout ammo.
 
Pistol length gas system is standard recommended gas system length for 300BLK, that is not your problem.
Don't think of it like it's a 223, this cartridge uses completely different pressures and powder burn rates.

First thing I'd do is try some different ammo. Hornady sometimes loads their stuff a little on the hot side for a semi auto. My Modern Hunter in 6.5CM does not like superformance ammo at all but if I run my handloads with the same bullet weight it runs 100% reliably.

If you're a handloader try 19.5gr of H110 under a 110gr V-max, it has run well and accurately in all my semi auto 300BLK rifles over the years.

Not all factory ammo is suitable for a semi auto, that being said I also wouldn't rule out that your problems could also be that you bought an NEA, if different ammo doesn't solve the problem have a gunsmith measure your chamber.
 
The primers dont look high pressure. Not sure why you would want to rule out the gas system since it seems 2 of the rounds started coming out of the chamber when the pressure was high
 
Can you load them from the mag and ease the bolt forward watching for them hanging up on the feed ramps or something weird happening?
A couple look like they have marks on the base from the bolt maybe they're hitting the ramp and pushing the bullet back a little which would increase your pressures.
Also try loading from the mag letting the bolt drop with the bolt release then eject them by hand, measure the cartridge before and after and see if anything is changing.
I would try different ammo and if the problem is still there have the chamber measured, not just headspace but properly measured, NEA is well known for out of spec parts and it wouldn't surprise me if there was some sloppy machining done.

porpoise said:
The primers dont look high pressure. Not sure why you would want to rule out the gas system since it seems 2 of the rounds started coming out of the chamber when the pressure was high
Click to expand...

Other than a way oversized gas port (which is very possible with NEA) there is nothing else that could be the problem with the gas system. Pistol length is standard length for all 300BLK barrels.
 
cr5 said:
Pistol length gas system is standard recommended gas system length for 300BLK, that is not your problem.
Don't think of it like it's a 223, this cartridge uses completely different pressures and powder burn rates.

First thing I'd do is try some different ammo. Hornady sometimes loads their stuff a little on the hot side for a semi auto. My Modern Hunter in 6.5CM does not like superformance ammo at all but if I run my handloads with the same bullet weight it runs 100% reliably.

If you're a handloader try 19.5gr of H110 under a 110gr V-max, it has run well and accurately in all my semi auto 300BLK rifles over the years.

Not all factory ammo is suitable for a semi auto, that being said I also wouldn't rule out that your problems could also be that you bought an NEA, if different ammo doesn't solve the problem have a gunsmith measure your chamber.
Click to expand...

I bought 4 different brands /mixed weights of ammo and will be trying it out tonight. I have also loaded up some 150 grain hornady FMJ with 296 and 4227 to test out.

I have also ordered an adjustable gas block and H3 buffer that I will be installing once it comes in. It could very well be a mixture of overloaded ammo and overgassing although if it was solely overgassing it would have created the same marks on all rounds. There is something else up.
 
cr5 said:
Can you load them from the mag and ease the bolt forward watching for them hanging up on the feed ramps or something weird happening?
A couple look like they have marks on the base from the bolt maybe they're hitting the ramp and pushing the bullet back a little which would increase your pressures.
Also try loading from the mag letting the bolt drop with the bolt release then eject them by hand, measure the cartridge before and after and see if anything is changing.
I would try different ammo and if the problem is still there have the chamber measured, not just headspace but properly measured, NEA is well known for out of spec parts and it wouldn't surprise me if there was some sloppy machining done.



Other than a way oversized gas port (which is very possible with NEA) there is nothing else that could be the problem with the gas system. Pistol length is standard length for all 300BLK barrels.
Click to expand...

I spent some time doing this lastnight with various rounds. I did have a small small around of setback with the hornady factory ammo. This occurred after letting the bolt fly home 3 times on the same round. I tried Remington, S&B and ppu and none of those moved one bit even after multiple times being rechambered.

If I have issues after this range trip I will be taking the barrel off and sending it back to nea for inspection.
 
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