Home Forums Hunting & Shooting AR and Tactical Rifles Over pressure and blowing primers after barrel swap

I removed a known good barrel and installed it into a new build, the new upper is a Spikes Tactical in FDE cerokote. Having done this dozens of times, it was business as usual. I used a vise, Geissele Reaction rod, and a torque wrench… fully assembled, I went to the range.. I installed the same Bolt carrier group as well. Groups were abysmal! 8”-10” at 100 yards and all over the place, recoil was stiff, bolt swipe was deep, and had 1 blown primer.. tried 2 Hornady factory loads and American Eagle. (Just a couple of each) even wolf steel case was showing high pressure. What caused this? My only thought is the barrel extension twisted. Upon case inspection, it seemed like the Hornady brass case mouths were contacting the lands possible.. ding marks.

same barrel same bolt carrier group that was once very good both removed and went into a different upper and went to [bleep]? maybe one or both of them uppers is miss made in the head spacing is wrong..?
I guess is the upper where you think pressure is high and it won't shot like [bleep] but... did you face the new upper? maybe take a bit too much off and set the barrel back? I would have thought the knot on the barrel it is been caught in the slot of the upper before it got too drastic.

The new upper is a Spikes Tactical and I have not squared the face. It’s factory original..

Is there way to measure the Spikes upper to check for that problem.. being short would definitely be an issue it would seem.

I'm kind of grasping at straws here thinking of things that have would have changed.
I guess you could install the bolt carrier group and measure from the face of the receiver down to the face of the bolt..
the obvious easy thing is if you can come up with go with no go gauges on the assembled product.

I’m going to try another barrel that’s a known good and go from there, only changing the one variable.

It sounds like the headspace is all wrong. Is it a chrome lined barrel ??

kwg

Assuming that all said in the OP is accurate, there must be a negative change (shortening) of the dimension from the HS shoulder point in the chamber to the bolt face as locked into the extension lugs in the new upper.

About the only way that might have happened is moving the barrel extension on the barrel, somehow, some way.

If the OP has a HS Go dimension gauge, which is minimum chamber, you can then see if the bolt will close on that HS Go gauge.................the chamber would have to be several thousandths below minimum for the primer blowing to occur.

But that would also make chambering nominal HS rounds difficult to chamber.(Overlength cases will crush 2-3 thou when the bolt slams closed.)

Pins on extensions do break occasionally, but that's surely not an everyday event.

I can't explain it any other way, if the bolt & barrel worked previously & were moved as a unit.

Gremlins..................LOL

MM

I am not sure how a new upper would changed anything as the bolt locks up in the barrel extension. As MontanaMan said, maybe something happened with the extension but to shorten the headspace, the barrel would have to be moved back against the shoulder more. Not saying that could not happen but I would not think that would be the case.
As far as factory ammo goes, most are a few thousandths under the go gauge length to insure chambering.
I will be watching to see what you find the problem to be.

Maybe the bullets are getting set back in the case during feeding.

Standard procedure when assembling an AR barrel into a receiver, is to blend and polish the barrel extension's feed ramps down into the M4 cuts of the upper receiver.
There is always a mismatch there.

Tony Rumore
Tromix

That's a possibility, easy to check....................he said he had the problem with several types of factory & that's usually crimped by hard though.

That could be more likely than the extension moving.

MM

Hopefully the op will get it figured out. You guys are pulling at straws here. Especially Tony Rumores post. Carry on though.. Its never not entertaining.. The OP didn't do something right. He needs to pull it apart and start over, if necessary. How much did he have to drink when assembling??? Is he using the same lower? Something is just not adding up.

Had issues like that with a 6ARC build. Throat length was incorrect, causing case necks to jam in lands, spiking pressure. Pulled bullet from factory case and chambered it. Would not go without a full bolt closure. Called the barrel manufacture, got a new one that works just fine.

My thought was everything was the same except the barrel and the brass is showing marks from the throat/leade? Sounds like the chamber may not have been cut properly but that is just a stab in the dark. Did the OP use Go- No Go guages to check headspace? Something isn't adding up for sure but not enough info for a complete diagnosis

I believe he said it was a known good barrel that he installed on a new upper. Which does not make any sense. Your question about no go and go gauge is definitely warranted. I think there are far too many guys just throwing schidt together without them. You guys are taking a risk, is all I have to say:

You shouldn't be using a reaction rod for torquing the barrel nut. You will get away with it, if you have a 7075 receiver, but the budget receivers are 6061 and many of those are made from extrusions.
The nut can and will drag the barrel extension with it when you tighten it up. That will side-load the indexing pin and either rock it over to the side, break it,
or more likely, crush the aluminum and widen the pin slot in your upper receiver. At that point, the bolt will no longer be in alignment with the lugs in the barrel extension.

You have gotten away with it for awhile, but you will eventually run out of luck and ruin a receiver.

Tony Rumore
Tromix

has the issue been found here?