AR-10 6.5 Creedmoor Over Gas Issues Using SRP FC Ammo?

[Brab6.5CM]

I am testing ammo in a new Aero Precision AR-10 in 6.5 Creedmoor. Accidently bought a box of FC 139gr Federal with small primers. All other large primer ammo (Federal, Remington and Hornady) I have shot so far functions great. The FC small primer is giving me high psi indications in the recoil and blow by. Also bulges or pops primer strike. See photos attached.

Anyone else having this kind of issue? Firing pin is smooth and correct length. Not sure what is happening.

 

[bfoosh006]

Sorry , What specific ammo ? Link please. ( I didn't think Fed sold a 139gr 6.5CM load )

I have had the Fed. 120gr AE pierce primers on one lot. The lot before was not an issue.

 

[Brab6.5CM]

I do not have original box. It was the 140 gr Berger shown below. When I pulled several bullets they were consistently closer to 139 grains so being OCD I labeled my plastic ammo box accordingly.

Talked to Federal and they said,”Currently we switched over to small rifle primers in that caliber, the earlier stuff had large 210 primers used, we switched for better accuracy and better standard deviations in our ballistics.”

Check it out on Federal website. I am tying to figure out it why it shoots so violently in my AR-10.

Gold Medal Berger 6.5 Creedmoor​

GM65CRDBH2

​

 

[bfoosh006]

OCD is fine , I understand now.

Do you happen to know your FP tip size ? And possibly the bolts FP hole size ?

What brand bolt ?

And it isn't often I see the primers anvil protruding from the pierced primer hole.

This chart has been around for as long as I can remember... so I really don't know if primer cup thicknesses have changed at all.

But it could be the thinner Fed SR Match primer cup is contributing to your issue ( along with the recoil issue )

The additional blowby could be from the venting pressure at the pierced primer.

 

[Brab6.5CM]

OCD is fine , I understand now.

Do you happen to know your FP tip size ? And possibly the bolts FP hole size ?

What brand bolt ?

And it isn't often I see the primers anvil protruding from the pierced primer hole.

This chart has been around for as long as I can remember... so I really don't know if primer cup thicknesses have changed at all.

But it could be the thinner Fed SR Match primer cup is contributing to your issue ( along with the recoil issue )

The additional blowby could be from the venting pressure at the pierced primer.

View attachment 7594477

 

[Brab6.5CM]

Yes Sir,

BCG is

Aero Precision .308 Nickel Boron Bolt Carrier Group​

Pin diameter is .075”, length is .036”
Bolt hole diameter is .079” using pin gage
(.004” difference)

 

[Expert684]

Measure firing pin protrusion with the bolt in the "battery" position. It may be penetrating the primer. Have you tried other ammo?

 

[whatsupdoc]

Where did you find all them primer punch outs?

 

[Brab6.5CM]

Measure firing pin protrusion with the bolt in the "battery" position. It may be penetrating the primer. Have you tried other ammo?

Pin length is .036” measured in battery from bolt ring face.

 

[Brab6.5CM]

Where did you find all them primer punch outs?

They were in the bolt and fell out when I pulled the pin.

Pin length is .036” measured in battery from bolt ring face.

All large primer ammo tested works great. Hornady, Federal, Remington weights from the 120 to 140gr.

 

[JPaul]

It may be the neck tension; or lack thereof. I had some SRP 6.5cm federal premium maybe two years ago that did the same exact thing, I can't remember the bullet but when the rifle auto fed the momentum caused the bullet to lunge forward. When I manually fed, the rifle stacked them. I addressed the issue to federal but never got a reply. I still have that ammo stashed somewhere, figured since it shot so good manually I'd save it if I ever got a bolt gun.

 

[Brab6.5CM]

Measure firing pin protrusion with the bolt in the "battery" position. It may be penetrating the primer. Have you tried other ammo?

I just realized that my reply’s to different post show up at the end of the thread. Think I am doing it right. Thanks for all the questions and suggestions. So far Federal is sending me another box of ammo. I will measure everything, OAL, seating depth, neck, chamber...

Sargent, thanks for the primer dimension chart. I am going to explore the Fed SR Match specs you mentioned.

 

[Expert684]

There is the possibility that the ammo is not suitable for your weapon. I have a DPMS 308 from the customs shop. Tried Hornady Super Performance 308 and it blew primers out of the pocket. Way too hot for the gun, and the gun was over-gased due to the hot load. My AR10s and I have three like mild loads. Hand loads and my 18" is 43.0 of Varget with a 175 SMK, and I have a gas regulator on it. Try another brand, weight bullet and same weight bullet.

 

[Expert684]

They were in the bolt and fell out when I pulled the pin.

All large primer ammo tested works great. Hornady, Federal, Remington weights from the 120 to 140gr.

Then I'd stay away from small primed brass. Everyone I know who I shoot with (F Class/FTR) bushing their bolts and turn the firing pin diameter down for SRP brass.

 

[Brab6.5CM]

There is the possibility that the ammo is not suitable for your weapon. I have a DPMS 308 from the customs shop. Tried Hornady Super Performance 308 and it blew primers out of the pocket. Way too hot for the gun, and the gun was over-gased due to the hot load. My AR10s and I have three like mild loads. Hand loads and my 18" is 43.0 of Varget with a 175 SMK, and I have a gas regulator on it. Try another brand, weight bullet and same weight bullet.

Copy that. I shoot F-Class. 178gr A-Max riding 42 grains of 8208XBR Kick started by a Fed large magnum primer. would never shoot that load in my gas gun!

 

[Brab6.5CM]

Then I'd stay away from small primed brass. Everyone I know who I shoot with (F Class/FTR) bushing their bolts and turn the firing pin diameter down for SRP brass.

Yea I have heard that and may go down that road if I can’t get large primer brass In the future.

 

[bfoosh006]

Do you still have the box of the offending ammo ? .. Specifically the Lot # ( typically, printed inside on of the end flaps. )

Be sure to compare the lot number to the new box prior to firing.

 

[Brab6.5CM]

No sir I do not. You see I have reloaded all my ammo for 35 years. Once I find a factory load that works I figure out the recipe and reload it. 6.5 CM is my new toy load which comes at this time of hoarders and shortage of components. Guess I will have to end the conflict with me ol .30 caliber stash! I will keep lot numbers from this point on.

 

[Saf3sh00t3r]

check to see if the ammo is too long for the chamber. Meaning it is jamming into the lands, that will cause pressure spikes/issues.

Had this issue with prime 6.5 ammo, all I did is reduce seating depth about .020 until it eroded enough where it wasn't needed.

Also, are you using a suppressor?

 

[svxwilson]

Get yourself a JP "high pressure" bolt. Bet that fixes the problem.

 

[Saf3sh00t3r]

Get yourself a JP "high pressure" bolt. Bet that fixes the problem.

Thats an expensive fix if his barrel is not headspace for it. That said a .062" pin diameter should help.

 

[MPD142]

I have a friend who experienced similar problems at a recent match shooting a LMT MWS with a gunsmith spun up 14.5 6.5 Creedmoor barrel. The 130 FGMM shot fine with the Large Primer, the Small primer ammo resulted in Popped primers and a gun that would not run. He's looking into a High Pressure bolt for it to see if that will cure his issues.

 

[Skunkworx]

This is why folks like JP sell a high pressure bolt. It’s the equivalent of a small firing pin in a bolt gun.

 

[msgriff]

Firing pin diameter to the bolts firing pin hole is larger than I've ever seen (0.004").

 

[Brab6.5CM]

check to see if the ammo is too long for the chamber. Meaning it is jamming into the lands, that will cause pressure spikes/issues.

Had this issue with prime 6.5 ammo, all I did is reduce seating depth about .020 until it eroded enough where it wasn't needed.

Also, are you using a suppressor?

Well good thoughts there. It’s factory ammo so I am guessing it was ment to shoot in a wide spectrum of chambers. Since I built this chamber to spec, one would think it would shoot most factory ammo. Pretty sure they design it that way. So now the question is does the small rifle primed ammo meet or exceed chamber pressure specs or is it unreliable cause it is designed with insufficient

 

[Brab6.5CM]

Well good thoughts there. It’s factory ammo so I am guessing it was ment to shoot in a wide spectrum of chambers. Since I built this chamber to spec, one would think it would shoot most factory ammo. Pretty sure they design it that way. So now the question is does the small rifle primed ammo meet or exceed chamber pressure specs or is it unreliable cause it is designed with insufficient neck tension? That is where I am testing now. Suppressed yes and no. Test dependent. I will look at bullet slide on chambering.

 

[KnightOfNee]

Yes Sir,

BCG is

Aero Precision .308 Nickel Boron Bolt Carrier Group​

Pin diameter is .075”, length is .036”
Bolt hole diameter is .079” using pin gage
(.004” difference)

That's your problem right there, dump that Aero bolt out of your carrier, their 308 bolt and pin are crap" I cant beleave they are still getting away with selling them", I had two of them, replace it with a JP High presure bolt which comes with a smaller dia firing pin. also if you have an adjustible gas block, adjust it for less presure if possible, the dwell time might be off and trying to fling the bolt back before enuff presure has vented from the rifle. Once the bolt is replaced make sure you check the headspace with a go and no go gauge, make sure you remove the ejector from the bolt carefully before using these tools.

If you are uncomfortable doing this sort of work, take it to a good smith that knows large frame ARs, not some local 18 year old tacticool wannabe know it all shit that seems to be at most of the gun store lately. but a real smith. These rifles can go bang in a very bad way.

 

[Brab6.5CM]

.308 bolt and pin issue. What is the proper measurement? Smaller diameter firing pin. Head space? Got that nailed. You for real or just trying to get my goat? Real smith USMC thank you.

 

[KnightOfNee]

.308 bolt and pin issue. What is the proper measurement? Smaller diameter firing pin. Head space? Got that nailed. You for real or just trying to get my goat? Real smith USMC thank you.

I'm for real, Just didn't know your background and making sure you stay safe. I had the same exact issue that you are currently having, was with Aero bolts and firing pins, the issues were with 308 Obermeyer chamber and Krieger barrel. problems with that rig are currently fixed. and that rig never has any more primer issues and that is using my favorite CCi Br2 which are a soft benchrest primer and really shouldnt be used in most autos.

 

[Brab6.5CM]

I have the Obermeyer chamber in a .308 F Class. Love it! just going to stay away from SRP brass for this 6.5. Will reload Starline LRP brass.