Help with overpressure...

[Cvitanic]

Hi all ! I need a little help to figure this out. We're developing a load with new powder we haven't used previously, so we did a test fire using nominal load from manufacturer on two different SIG ARs. One was totally normal and the other one shows signs of overpressure (cratered primers, enlarged primer pockets, bolt head marks on case head).

Why is this happening on a single rifle and not both ? Could be related to headspace issues ? Lubricant seeping into the chamber ? Any ideas ?

"Good" AR:

"Bad" AR:

 

[cas6969]

You have primer flow but no obvious (terrible) flattening of the primers. I assume you're using standard small rifle primers?


(So much going on in those pics. You also seem to have some huge variation in primer seating depths? (Some sticking way out, but not hammer flat, which sometimes means low pressure, not high. Don't think it would in the rifle though))


Some people might cringe at the idea, but I'd swap bolts/carriers between rifles and see if the issue swaps with them.

 

[Cvitanic]

You have primer flow but no obvious (terrible) flattening of the primers. I assume you're using standard small rifle primers?


(So much going on in those pics. You also seem to have some huge variation in primer seating depths? (Some sticking way out, but not hammer flat, which sometimes means low pressure, not high. Don't think it would in the rifle though))


Some people might cringe at the idea, but I'd swap bolts/carriers between rifles and see if the issue swaps with them.

Thanks ! We are using Ginex 4,5 Small Rifle Primers. When you mention a variation in seating depths, you refer to the "bad" ones ? I believe those were seated correctly when primed, and due to primer pocket expansion they loosened (on some it would only take a light shake and they would fall off).

Why would it be a bad idea to swap carriers ?

 

[B y r o n]

You didn't mention which model Sig rifle you are comparing. Are these DI or piston versions?

 

[cas6969]

Well if your primers are falling out, it's not a "non AR primer" problem.

Taking a better look at the other pictures now, I see the case head deformation I missed before.

Maybe a silly question but, they're both 5.56 chambers? Not one .223? (most of the blown primers I ran across over the years were people shooting 5.56 in .223 chambered AR's. I don't think they're very common any more though)

 

[Cvitanic]

You didn't mention which model Sig rifle you are comparing. Are these DI or piston versions?

Hi ! Both are piston operated SIG 556.

 

[Cvitanic]

Well if your primers are falling out, it's not a "non AR primer" problem.

Taking a better look at the other pictures now, I see the case head deformation I missed before.

Maybe a silly question but, they're both 5.56 chambers? Not one .223? (most of the blown primers I ran across over the years were people shooting 5.56 in .223 chambered AR's. I don't think they're very common any more though)

They're definitely 5,56x45 SIG 556 systems.

 

[DeathBeforeDismount]

With sig it's a feature, not a defect.

 

[m1match]

Like cas6969 said, both of your rifles look like you're getting overpressure signs on the cases. Your "good AR" cases look like they have ejector marks on the case head, just not as bad as the "bad AR". Your bad AR has very definite ejector marks which is definitively overpressure. If I were you I'd back off the powder charges before shooting that powder again.

 

[B y r o n]

Hi ! Both are piston operated SIG 556.

Are they both set to the same setting? If one is set for suppressor/less gas and the other is set wide open that could be the difference.

 

[Cvitanic]

bad as the "bad AR". Your bad AR has very definite eje

Like cas6969 said, both of your rifles look like you're getting overpressure signs on the cases. Your "good AR" cases look like they have ejector marks on the case head, just not as bad as the "bad AR". Your bad AR has very definite ejector marks which is definitively overpressure. If I were you I'd back off the powder charges before shooting that powder again.

Thanks, we'll run a pressure test to be sure. I'll post the results.

 

[Cvitanic]

Are they both set to the same setting? If one is set for suppressor/less gas and the other is set wide open that could be the difference.

Good point. Will check again, thanks !

 

[Yondering]

It's very likely that one rifle has a shorter and/or tighter throat than the other. The chamber itself may be a little tighter as well. That's all it takes to cause the differences you've shown in your pics.

If you're trying to develop a load that'll work in any 5.56 rifle, then develop the load for the one that's giving you pressure signs above.

But if you want both to perform similarly with the same load, I'd find a good gunsmith to look in the chamber and determine what's going on, and whether he can fix the tight one. (Keeping in mind that it might be more that one of your rifles isn't "tight" relative to intended specs, but the other is loose, resulting in lower pressure in what would normally be an over-pressure load.) It may be that one needs to have the chamber polished a bit, or the throat lengthened, but it could also be that you've got a lot of carbon built up in one of those. A hard ring of carbon can build up in the throat area, driving pressures higher and hurting accuracy; that should be cleaned out but it can be difficult to do so.

 

[Cvitanic]

It's very likely that one rifle has a shorter and/or tighter throat than the other. The chamber itself may be a little tighter as well. That's all it takes to cause the differences you've shown in your pics.

If you're trying to develop a load that'll work in any 5.56 rifle, then develop the load for the one that's giving you pressure signs above.

But if you want both to perform similarly with the same load, I'd find a good gunsmith to look in the chamber and determine what's going on, and whether he can fix the tight one. (Keeping in mind that it might be more that one of your rifles isn't "tight" relative to intended specs, but the other is loose, resulting in lower pressure in what would normally be an over-pressure load.) It may be that one needs to have the chamber polished a bit, or the throat lengthened, but it could also be that you've got a lot of carbon built up in one of those. A hard ring of carbon can build up in the throat area, driving pressures higher and hurting accuracy; that should be cleaned out but it can be difficult to do so.

Thanks for your input. We ran some pressure testing and results are all over the place, probably due to grain size unevenness.

 

[XLR308]

Thanks, we'll run a pressure test to be sure. I'll post the results.

Could you list what powder you are using and what your sized cases are compared to fired dimensions in regards to shoulder bump for both rifles chambers.
Also a pic of the primers factory containers being used since the Ginex 4.5 are both a designation for small pistol and small rifle but you didnt include the rest of the part number.

 

[Yondering]

Thanks for your input. We ran some pressure testing and results are all over the place, probably due to grain size unevenness.

View attachment 8127753

Interesting. Was that pressure test done in a separate test barrel, or a strain-based measurement of one of the two AR15s mentioned above?

There are a number of possible causes for that much variation in pressure. Powder charge weight is one (I wouldn’t blame variation in size if individual powder grains, that should even out within any given charge), but case volume, neck tension, seating depth, etc all play a part.

Was this pressure test done with mixed brass maybe? Or just very poorly controlled powder charge weights?

It’s also a strong possibility you’ve got something going on with case necks sticking to the bullets; that can really throw a lot of variation into your loads. For example if the cases were tumbled in stainless pins, and then they’ve been loaded for a few months or more, you might be seeing results of cold welding between necks and bullets. If you pull a few bullets you might see evidence of this, presenting as spots of brass stuck to the bullets. If you try seating the bullets .010”-.015” deeper you may hear and feel a sharp crack as the bullet starts to move, that’s another sign of this issue.

Hope that helps. Interested to hear more details about this.

 

[Cvitanic]

Could you list what powder you are using and what your sized cases are compared to fired dimensions in regards to shoulder bump for both rifles chambers.
Also a pic of the primers factory containers being used since the Ginex 4.5 are both a designation for small pistol and small rifle but you didnt include the rest of the part number.

Thanks for your reply. Powder is specific for 5,56x45 from FM Granada in Spain. Pretty crappy stuff in terms of grain size (see pic below).
Primers are Ginex 4,5/3-P3.

 

[Cvitanic]

Interesting. Was that pressure test done in a separate test barrel, or a strain-based measurement of one of the two AR15s mentioned above?

There are a number of possible causes for that much variation in pressure. Powder charge weight is one (I wouldn’t blame variation in size if individual powder grains, that should even out within any given charge), but case volume, neck tension, seating depth, etc all play a part.

Was this pressure test done with mixed brass maybe? Or just very poorly controlled powder charge weights?

It’s also a strong possibility you’ve got something going on with case necks sticking to the bullets; that can really throw a lot of variation into your loads. For example if the cases were tumbled in stainless pins, and then they’ve been loaded for a few months or more, you might be seeing results of cold welding between necks and bullets. If you pull a few bullets you might see evidence of this, presenting as spots of brass stuck to the bullets. If you try seating the bullets .010”-.015” deeper you may hear and feel a sharp crack as the bullet starts to move, that’s another sign of this issue.

Hope that helps. Interested to hear more details about this.

Hi ! Thanks for your reply.
Pressure testing was done on an EPVAT system. Powder charges have proven consistent so I would put my finger on irregular grain size (see photo), since powder is measured by volume, not weight. Brass is brand new never fired, freshly loaded; so that would rule out bullet welding.

We have another powder (Rheinmetall-Denel) from South Africa which we'll test to rule out the powder variable. Will post results as soon as we do the firing test.

 

[XLR308]

Thanks for your reply. Powder is specific for 5,56x45 from FM Granada in Spain. Pretty crappy stuff in terms of grain size (see pic below).
Primers are Ginex 4,5/3-P3.

View attachment 8128512View attachment 8128514View attachment 8128515

Wow just a random handful of sand is more uniform than that.
Even If you are precisely masuring your charge wieghts i wouldnt think you are getting an even and consistent burn rate based on volume density.
Just my opinion but would scrap that powder and if possible see if you could be refunded unless it was free.

 

[Cvitanic]

Wow just a random handful of sand is more uniform than that.
Even If you are precisely masuring your charge wieghts i wouldnt think you are getting an even and consistent burn rate based on volume density.
Just my opinion but would scrap that powder and if possible see if you could be refunded unless it was free.

Wasn't free, quite the opposite... but it's what was available in the market at the time. Most likely we will sift and classify accordingly to get more consistent loads. I agree it's crap but when life gives you lemons...