Over pressure in AR .204

[Hal]

I'm not a AR guy but as I walked past the used rack the .204 caliber caught my eye.
It turned out to be a Les Baer AR15 in .204.

I picked up about 50 rds of factory new Dogtown 32 gr ammo along with about 50 rds of once fired Dogtown brass.
I cleaned the barrel, mounted a scope and waited for the wind to die down.
Sunday afternoon and the wind was still blowing 12-15 mph. 35*
I shot 3, 3 shot groups before I froze out . The first 3 shot was just under 1/2". Great for a AR with a 61/2 lb trigger.
I looked at the cases and the primers were flat and base of the shells had two marks on them across from each other. More than a smear mark. I thought maybe the AR was rough on brass.

Next time I shot the rifle it was dead calm 70*. I I bought fired 5, 3 shot groups . Two groups were under 1/4"
The other 3 were 3/4" to 1"
I put a brass catcher on the AR, when I dump out the brass, a primmer fell out of a case and a few primers had a pin hole in the bottom of the firing pin indentation, some case had a small crater in the bottom of the firing pin indentation. This brass was fired on a 70* day.

I looked at the once fired factory Dogtown brass that had been fired in a factory Savage bolt gun.
It was normal, the primers were still rounded on the corner and the firing pin indentation was normal.

I bought a box of Hornady 32 gr V-Max Superformace and fired 2-3 shot groups, one around 1/2", one just under 1". Temp 65*
The primers are flat and no holes in the bottom of firing pin indentation. But still a hot load in my rifle.

Where do I start first to find the cause of high pressure in my AR with factory ammo.
Probably should take a chamber cast.

Thanks for any help. The AR is 15 years old and factory except for a heavier trigger.
Hal

 

[Coyotefurharvester]

Clean the chamber/barrel, like new clean. Make sure it is dry, find the bolt face to lands. Check with go-no go.

 

[jelenko]

My first thought is to check the throat for a carbon ring. With this being used by a previous owner, there could easily be a carbon ring if they didn't keep it clean.

 

[Hal]

I measured some of the brass using a micrometer.
As near as I could measure, the Dogtown 32gr .204 ammo when fired in the bolt action Savage expanded,at the base.003. The same ammo,when fired in my AR expanded about .0035 or a half thousands more.

Hal

 

[JB10]

Maybe a short throat in the AR causing pressures to spike if jamming bullet when loaded?

 

[L.Sherm]

My guess if it was chambered with a saami reamer it isn't a short throat problem they have .100 freebore.

 

[L.Sherm]

Measure a piece of the unfired Hornady and Dogtown brass with a comparator, will have to pull the bullets might be a headspace problem as in to much on the brass with the AR especially if Hornandy has the ammo loaded hot which i have seen before.
Then measure fired brass with the comparator

 

[spencerhenry]

adjustable gas block?

 

[Hal]

I used sharpie and colored one loaded Dogtown round and one Hornady round.
Neither one showed any rifling marks when chambered. The 32 gr bullets are very short. I'll have to find some longer bullets and a compparator.
I'm cleaning the barrel tonight with Butches bore cleaner and JB.
I'll have to find some cerosafe and make a chamber casting.

Hal

 

[243winxb]

Bullet setback on chambering?

The guess made on this 204.

 

[Hal]

Spencer
No it does not have a adjustable gas block.

Hal

 

[Hal]

243
WOW that sure looks scary.
Learn more everyday. Thanks for sharing.
Is there any warning signs or do you have to chamber a round and eject it before firing to see if the bullet is moving.

Hal

 

[spencerhenry]

I had a brand new ar-10 that would trash brass like that, the gas port was so oversized that it was opening the bolt before the pressure had dropped. Every case head was expanded with a primer falling out every now and then.
It was a known load that worked great in a couple of bolt rifles. I sent it back to Armalite, they at first told me it was because I was using handloads. I ran 20 rounds of Black Hills ammo through it, same thing case heads expanded and loose primers. Armalite bushed the gas port down and sent it back, it was better but still not there. I modified the gas block to make it adjustable, cut the gas way down and gradually opened it up until I got reliable functioning. No more expanded case heads and destroyed brass.

 

[jpjulian]

Being a used rifle, I'd first suspect a heavy carbon ring. If that isn't the case I would contact Les Baer. The problem could well be why it was on the used rack.......

 

[243winxb]

chamber a round and eject it before firing?

Bullet setback? Measure COL before and after chambering.


Post #13 good info. I would try to return gun to seller. End of problem.

The factory ammo choice of powder may be wrong for an auto loader?

Hornady link.-

Due to the longer duration of peak pressure produced by Superformance™, the post peak/declining port pressure at common carbine and mid-length gas port locations is still higher than that produced by standard propellant. This has a tendency to flood the system with a larger volume of gas, at a higher velocity, that tries to open the bolt of the gun too fast. It’s a timing issue. The cartridge case is still swollen from the application of pressure during firing while the gun is simultaneously trying to extract the cartridge case before it has had an opportunity to settle back to its original size, or more simply: the gun is still in the process or firing while it’s trying to extract the cartridge case.