After reading John McPhee’s AAR i’m seriously questioning my purchase.
This is what i’m talking about:
The Course: Training Day 2
Before td2 started, we had a weapon down from the day before. As I beat the bolt apart the story started to unfold. The cause was a LaRue OBR, which was having issues with Federal Gold Medal Match 175gr loads from the day before. This OBR was brand new (purchased and used for the course) and had major over pressure issues. Because the weapon had too much chamber pressure, each round was blowing out the primers into the Bolt face. Generally, any type of blown primer is an ammunition issue; however, the pressure from the chamber was also deforming the brass so bad that brass from inside the bullet casing was bulging out of the hole where the firing pin touched the primer. (See Pic 1). The pressure was so great it was pushing the primer pieces into the firing pin hole. This eventually jammed and locked the bolt up to where the weapon no longer functioned. There was about 5-10 small perfectly shaped primer pieces exactly sized to fit through firing pin hole. The OBRs are known to have over pressures issues. Nonetheless, it ran Mil-Spec M118LR ammunition just fine, due to the ammo being ruggedized with harder primers and thicker casings. This is something for future students to consider when bringing OBR’s to this course.
The FIX is to lower the chamber pressure to a more tolerable pressure that common ammunitions can shoot. Yes, this rides the line of accuracy versus less chamber pressure and the line between accuracy versus reliability. One student commented, "I wish I knew more before I bought this OBR and knowing what I know now, I would not have bought a Larue OBR.” I personally always go for reliability because if the gun doesn’t go boom why carry it. Also, I will sacrifice accuracy first because I would rather have a Reliable 2MOA gun than a .5 MOA that doesn’t shoot. Dually noted about this weapon’s failure is ammunition. The LaRue OBR works best with MIL-Spec Ammo, not everyone has access to Mil Spec ammo, and I always bring some for this reason. Hardly seems fair for the guy who bought a high-end rifle to end up not participating due to these kind of problems. Gun and gear manufactures watch your products because I’m writing AARs for every class. "Your manufacturing or production problems should never be the customers!”
I have a PredatOBR on order of course, but at a good price I have been stocking up on Federal 175 grain GMM and am only planning on running premium ammo from Federal, Hornady and Black Hills in 168 and 175 grain in the rifle. I’m also only using the rifle for long range shooting 1,000 plus and possibly some elk hunting.
Is the above issue common in other high end .308 gas guns with tighter chamber specs designed for precision (LRP-07, GAP-10, Noveske, KAC ECR) or is this something that has been happening more with LaRue rifles? I know a lot of different schools have run OBR’s and this is the first I’ve heard of this issue. I also thought the Xtraxn chamber fluting would help alleviate an issue like this somewhat…?
I take it this was factory loaded ammo? With that said, I recently got my hands on a precision rifle, 700 action, Krieger barrel, Bell & Carlson stock in .223, custom built for another fellow and he’d never shot it. I had something he wanted real bad! It came with 10 boxes of Hornady Match 75gr ammo. 2 boxes were a different lot than the other 8. I started breaking it in with the 2 odd ball lot. Well and good and sub-moa with this factory ammo. Got into the other lot, in the first 10-15 I was seeing some pierced primers and this lot wasn’t supposed to be as hot as the first lot. What wasn’t pierced were cratered like you see in over pressure conditions. No other pressure signs and a strong action, I continued. A couple rounds later had a mis-fire, light primer strike. Hummmmm?..Fired the second hit, a couple more shots, another light strike, then another, then it would not fire at all even with 3 or 4 hits. Well crap, probably busted a firing pin. Got her home, tore it apart, firing pin looked fine. Cleaning out the bolt, found little brass donuts from the pierced primers blown back into the bolt just as you did. Did some research and found others had had the same issues with Hornady match in AR! From what I could tell they used a bad batch of Winchester primers. So, it’s not just AR’s that can have this happen and a warning for when we see pierced primers, it could be the primers themselves and no matter what rifle type, (or brand of ammo it appears) it may cease to function…O.L.
I hosted McPhee for his “Heavy Carbine Marksmanship” course 3 weeks ago here in IL and every 7.62 LaRue in the class (5 total) displayed the same over-pressure issues except for one. All with factory loaded ammo.
I shot M118LR thru my 7.62 PredatAR and didn’t have any stoppages but each of my fired casings showed bulged primers. Another student with a PredatAR had his gun completely lock up on TD1 due to a popped primer lodged inside of the cam pin hole. He was running 175gr Fed GMM. Luckily he happened to have 500rds of LR that he finished the class with without any further stoppages but it also showed bulged primers.
John’s conclusion was that even if it runs with M118LR it is still way over-pressured and will burn out the chamber much sooner than it should (approx 2,000-2,500rds max). John believed that this is exactly what happened to the gun Larue submitted for the Mil evaluation that didn’t finish. The chamber was toast at 2,000rds.
I don’t know about any issues with the 7.62 guns specifically.
There are two possible primer related issues Blown primers
The whole body of the primer is loose or comes out. Usually ends up jamming the trigger.
Causes:
Over pressure ammo
Tight chamber / Tight bore
Soft brass / Loose primer pocket
Over gassed action
Early extraction
Pierced primers
A small chunk of primer is extruded into the FP hole in the bolt. Usually ends up jamming the firing pin.
Causes:
Over pressure ammo
Tight chamber / Tight bore
Primer cup thickness / strength / non mil-spec primers
Loose fit between the firing pin hole and the firing pin
With all the interrelated causes here, it’s more difficult to nail down the specific reason.
Completely separate issue. The over-pressure issue has been present since day one with the OBRs with their original Lothar barrels. McPhee seemed to think that it was the size of the gas port in conjunction with a few overly tight tolerances in the chamber, bolt face and a couple other places inside of the gun.
Just another data point: KevinB at KAC has stated before that FGMM brass is too soft for reliable use in semi auto rifles. I don’t know if this has changed in the last couple of years, but this is the reason I stayed away from the stuff when I had my SR25 EMC.
it is kind of curious why he jumped to the conclusion popped primers were ammo issue. assuming factory ammo and not somebody’s hot reloads, my immediate thought would be the chamber. when you have 5 guns doing the same, it’s obviously the chamber.
it’s not clear how it could be related to the gas port either.
and… i am extremely skeptical that hot rounds will drop the barrel life on a 308 from 8000+ to 2000-2500 rounds. i would love to see the firing schedule for that test. it’s not a belt fed.
and ffs, if you think rounds are “way overpressure” why would you continue shooting them?? or allow people in your class to continue shooting them?
He never said that he though it was an ammo issue. He said it is a gun issue. Everyone in the class was running premium factory loaded ammo. The biggest issues with popped primers/bulged primers appeared to be with factory Fed 175gr GMM. Factory M118LR functioned but still showed heavily bulged primers
Too much gas in-conjunction with the other mentioned tight tolerances.
He stated that the OBR that LaRue submitted for the Mil trial dropped out right around 2,000rds because of throat erosion and that was the reason it failed the trial.
Again, it’s not an issue with the various factory ammo that he has seen run thru the 7.62 LaRue guns in his classes. He said the issue is with the guns and he has not had a LaRue OBR 7.62 detonate in any of his “Heavy” classes. He said that he has seen the identical issues in every one of his “Heavy” classes with LaRue 7.62 guns. The over-pressure issues cause primers to pop and the guns to lock up due to those popped primers ending up causing the stoppage.
He also said that he considers Mark LaRue a friend and has no “Agenda” with Mark or his company, and is only reporting what he has seen in his classes.
ok thanks for the clarification. perhaps i misread the OP which I thought was quoting McPhee’s AAR when he wrote “Generally, any type of blown primer is an ammunition issue;” and went on to say it was soft brass and primers. The 3rd post in this thread seemed to continue and support that supposition, so that’s why I responded.
if your problem is “overpressure” or “blown primers”, how could too much gas affect that? “too much gas” is often a contributing factor to extraction issues when the bolt unlocks before the pressure subsides, but I still don’t see how it could cause overpressure in the first place, or blown primers, or be relevant to this conversation. Thanks for the education.
my last point was not ammo or gun related. it’s a safety issue. it’s a liability issue. the fact that he hasn’t witnessed a particular model of rifle blow up is nice, but this is one of those cases where after one does blow up and somebody gets hurt, everyone will look back on this and say it was a really really dumb idea to keep shooting and that there were plenty of warning signs.
I don’t know Larue or Mr. McPhee and have no agenda either. (though I have plenty of Larue product, dillo dust and bumper stickers)
To be clear, now that I think about it more I don’t think McPhee ever specifically mentioned the LaRue guns being “Over Gassed”, that may be me reading too much into what I observed at the class. He specifically said there was an “Over-Pressure” issue inside of the LaRue guns causing the problems with the ammo and the throat/chamber area of the guns.
That would be interesting to find out. I don’t have either of my Larue 7.62s anymore to help. Both of mine had smooth chambers, but I’m pretty sure they added some sort of spiral fluting to them later on. I assumed it was to help out some of their running issues?
My 7.62 PredatAR has the XTRAXN chamber treatment as did every other LaRue gun in the class that was showing pressure issues. The only one that didnt show any issues was a 2+ year old/pre-XTRAXN OBR.