JP upper + Factory Ammo= overpressure?

Suppressed in SD

Suppressed in SD

Member
I have a JP 260 Rem upper. Lately I've been blowing out primers. I don't reload (yet) so I've been shooting 2 different kinds of Federal premium. Last year I shot several hundered rounds of 140g Sierra Gamekings without a problem as well as a few boxes of other stuff. 0 primer blowouts. This fall I got my hands on several boxes of 120g Nosler ballistic tips. They shot a little better so I kept with them. Had a primer blow out and jam up my gun deer hunting and since have a many more do the same thing. In fact, I may blow out 3-4 primers in 20 rounds. My gunsmith (who is also a competative long range shooter) looked at the brass and he says "high pressure". Sent the ammo back to Federal and they tested it and say it's normal pressure and velocity. Gunsmith says the the chamber, throat and barrel are all normal and clean. Only thing he can find is the throat is relatively short as you would expect from a manufacturer going for the utmost accuracy. Recently Ive had the 140g rounds blowing primers too. Ive shot more than a hundred without a problem. Am I missing something here? So now here's the kicker. I call JP and they say its no surprise that I'm blowing primers as their 260 Rem was made with the idea of handloading and factory ammo is too hot. He tried to explain that factory 260 ammo is made for bolt guns and a gas gun isnt able to handle that high of pressure. What? Why does that equal blow primers? Is he saying that the throat is cut too short for factory ammo? Why else would the pressure be high and blow the primer? I really need some help here. I can't stand having an unrealiable gun.
 
Originally Posted By: Suppressed in SD
"I call JP and they say its no surprise that I'm blowing primers as their 260 Rem was made with the idea of handloading and factory ammo is too hot. He tried to explain that factory 260 ammo is made for bolt guns and a gas gun isn't able to handle that high of pressure.

What?

Why does that equal blow primers? Is he saying that the throat is cut too short for factory ammo? Why else would the pressure be high and blow the primer? I really need some help here. I can't stand having an unreliable gun.

I second that motion... WHAT???

JP has sent you out on a snipe hunt!

You have a chamber problem - that is the long and short of it!!

The pressure standards for the 260 Rem are EXACTLY the same as the pressure standards for the 221 FireBall.

Maximum Average Pressure (MAP) = 60,000 PSIA
Maximum Probable Lot Mean (MPLM) = 61,500 PSIA
Maximum Probable Sample Mean (MPSM) = 63,800PSIA

For the 260 Remington AND the 221 FireBall (and a lot of others, it is one of the basic standards)

JP is jerking your chain.


.
 
Federal pressure tested the 120g noslers that I was having so much trouble with and they got anywhere from 56,528 to 59,920. So evidently I'm creating much higher pressures in my chamber to pop those primers. Now what? My gunsmith suggested running a reamer down and lengthening the throat a little. Is that my only option? That or roll my own lower powered loads?
 
If they arent going to make it right i would lengthen the throat a little and be done with it. Pretty crappy they blew you that line of BS.
 
i had the same problem with an ar10 years ago. my bolt action handloads shot great in both the bolt rifle and the ar10, BUT in the ar10, the case heads would be expanded and the primers would blow out or fall out later. i sent it back to armalite and they said there was nothing wrong with it and gave me a very similar answer that you got. they also said that i should not shoot reloads and that that was the problem. so a friend that worked for the fbi gave me a box of ammo that they use (factory ammo). same problem. i sent it back to armalite twice, they "bushed down" the gas port and sent it back. it was better but still not right. what i determined was that there was too much gas going to the action and the bolt was opening before the pressure had dropped. i had brass smears from the ejector, expanded case heads, and blown primers. for several years it sat in the safe, then finally i modified my gas block to make it adjustable.
now i have NO problems with destroyed brass and no loss in reliability or accuracy.

dont know if that is what is going on in your situation, but maybe?
 
I have an adjustable gas block and have the gas turned down as low as I dare. I thought maybe that would help but it didn't. In fact, that was the only other suggestion by JP. Guess I gotta call them again.
 
as low as you dare? what does that mean? you can shut the gas off all the way it wont hurt anything, just wont cycle.

but, if the gas is low already, then that is not the issue.
 
When you write them, tell them that you have shared your problems with other shooters on several internet forums and your forum friends are also interested in how they respond.

In the description of the upper in their sales material, was there anything written about how this upper was designed for reloads only and not SAAMI specs?

JP usually has their stuff together, this one surprises me.
 
Maybe the neck/bullet junction is being jammed at the end of the chamber. That would send the pressure up. I had a similar problem and it would give me head separation. Perhaps if I had used regular primers vice mag primers I might have had blown primers also.
 
There is another similar thread in ARs because I also wanted to find others with this same upper and share information with them. I don't look through every predator masters forum, just the ones that interest me. I assume that most others do the same. I want to expose this problem to the most people I can in hope to figure out a solution.
 
Originally Posted By: 6724as low as you dare? what does that mean? you can shut the gas off all the way it wont hurt anything, just wont cycle.

but, if the gas is low already, then that is not the issue.

I use this for coyote hunting so it must cycle. It's throwing brass over my right shoulder, any lower and I'm afraid it won't cycle. I couldn't turn it down lower if I wanted. The stupid tiny set screw stripped the last time I tried to adjust it. Luckily it ended up right where I wanted it.
 
Originally Posted By: Clay34

In the description of the upper in their sales material, was there anything written about how this upper was designed for reloads only and not SAAMI specs?

JP usually has their stuff together, this one surprises me.

Bought it off of gun broker. Don't have any sales material. Should be listed on their website though.
 
Originally Posted By: MGYSGTMaybe the neck/bullet junction is being jammed at the end of the chamber. That would send the pressure up. I had a similar problem and it would give me head separation. Perhaps if I had used regular primers vice mag primers I might have had blown primers also.

So how did you fix it?
 
I have been researching the grendel and found this, could be your problem.

The factory ammo is engaging the lands because the Midway AR-Stoner barrel wasn't cut with a Grendel reamer, which is a SAAMI spec'd, open-source chamber now. We actually see this a lot on the Grendel Forum. A Hide & Grendel member who does barrel work used an ogive comparator, and saw anywhere from .020"-.030" of COL shorter on the AR-Stoner barrel than his Shilen Grendel barrel.

All factory ammo, from 90gr TNT's, to 129gr SST's have fed and extracted fine in my Grendel chamber, and I can actually load out to 1.300" with the 129gr SST in my gun. I just wish everyone in the industry would use the SAAMI chamber, rather than a derivative of a chamber that was built around the 123gr SMK. You don't know what you're getting with many of the barrels on the market, which is why AA was so insistent that barrel-makers adhere strictly to the Grendel chamber diagram. It makes sense to have one datum, so all the factory ammo makers know where to seat their bullets with different ogives.

Right now, it's hit and miss with barrels that weren't cut with a Grendel reamer. Another thing to think about is start pressure. With the projectile jammed into the lands, you're most likely in territory over 50,000psi chamber pressure. We have seen cases where guys are getting soot flow out the primer pocket, wondering why their "Grendel" is doing that with factory 123gr AMAX, only to learn the barrel was cut by an unknown source, or someone who thought their chamber was better, that didn't want to be held to the QC standards spec'd by the Grendel chamber dimensions.

In the 6.5 Grendel Reloading Handbook, this aspect of chamber variants is addressed in detail, to include chamber drawings. Know thy chamber, and load/feed for it.
 
I appreciate the information. I know very little about bullet seating and the like. Thus my reliance on factory ammo. The more I read about this the more I think that my bullets are jammed into the lands. When I charge a live round and then remove it, on close inspection there are 2 very light curving scratches on the lead. I assume this is from the lands. Would this ammo be salvageable if I seated the bullets deeper? Or should I reame the throat a little?
 
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