6.5C pressure issues

Bigshooter42

Bigshooter42

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Minuteman
Dec 7, 2011
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DFW TX
#1
Hi guys,

Need to consult the knowledge base here. I'm not new to gas guns but new to the AR10 platform and am having some pressure issues with my 6.5C build and hand loads. Looking for some direction on how to correct this so I can get to shooting.

Build Detail:
Faxon Firearms 16" 6.5 Creedmoor (mid length gas)
I purchased the factory headspaced matching bolt from Faxon as well
Superlative arms bleed off gas block
Springco orange buffer spring with 5.3oz buffer

Load:
Varget (I know this is not ideal powder but I am converting from 308 to 6.5 and have a lot of Varget to use)
Started my load at Hodgdon's recommendation of 36.0 and worked up to about 38.5
Hornady brass with 120gr ELD match bullets and Winchester primers

Issue:
Even at the lower end of the charging scale I am getting cratering primers and bad ejector swipes. I have turned down the gas block to only use enough to cycle the action. I had similar results with Hornady factory ammo (both the 120 ELDs and 129 SSTs). I recently added the springco and heavier buffer thinking this would correct the issue. It did get better because before that change my primers were flowing badly, but they are still cratered and even blew one out at the 38.5 load.

Any recommendations on where to go from here would be much appreciated. I am not sure if it's my barrel, the powder choice, or lack of a heavy enough buffer to get this working. I do have some H4350 but I was going to use that for my bolt gun loads.

Thanks!
 
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#2
Far left is 38.0gr of Varget and far right is 39.0gr and where I stopped. The one with the silver primer in front is before the buffer and spring change. Much flatter primer with flow.
 

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#3
Birddog6424 said:
I doubt buffer weight has anything to do with it. It's not so much a cycling issue as a pressure issue.

Have you measured your brass before and after firing? I saw this once before and it was a poorly cut chamber. Also a go no go gauge check wouldn't be out of order. It's rare that headspace is an issue, but it happens.
Click to expand...


Looks like 1.548 to shoulders before and checked a few that averaged 1.556 so .008 increase.
 
#4
Why the hell would they put a carbine length gas system for a Creedmoor? Your port pressure is probably ridiculously high. I would be surprised if you ever got it to run with a carbine length gas system. Good luck fucking around with that.
 
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#6
Ideally it would have an extended length, like +2" over rifle length, gas system. You need a full weight carrier and the heaviest buffer / stiffest spring combo you can find.

The issue is dwell time, the bolt is unlocking early, while still under pressure. Hence the wipe. The cratering is a pressure problem, use less Varget.

Good luck.
 
#8
Bigshooter42 said:
My bad it’s actually a mid-length gas.
Click to expand...

That's what I was thinking, but that is still a problem. Get rid of that barrel and get an actual rifle length system, +2 gas system. That thing is going to cause you problems for a long time, in fact, you may never get it stabilized. The action on that rig is going to be unlocking early, which can cause a whole host of malfs.
 
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#9
Birddog6424 said:
It's only a 16" barrel, so dwell time isn't ridiculous. There are a lot of AR10' s in 308 that run mid length 16" barrels. My son in laws DPMS LR308 is like that and runs like a champ. Though I agree with you guys that I would rather have an 18 or 20" rifle length gas or a +2 length on longer.

Whats the fill capacity on 38grs of powder? That's not a lot of powder in that case. Maybe your getting some pressure spiking?

Edit: Went and answered my own question. At 38grs you're at 73% case fill. Which opens the door to possible pressure spiking. I had this conversation with one of the Berger ballisticians this fall regarding my 300 Norma, and how that empty space effects the pressure curve. He ran some really cool simulations for me on something that seemed to approximate Quickload, but it was Berger's in house program. Not saying it's your problem, but the potential is there.

You are shooting light 120gr bullets, so not much case intrusion. You could easily run that over 43 to 44grs of H4350. Buy a lb of 4350 and shoot it at 42.5grs and see what happens.
Click to expand...

Makes sense. I’ll try my luck with the 4350.

Thanks all.
 
#10
Birddog6424 said:
It's only a 16" barrel, so dwell time isn't ridiculous. There are a lot of AR10' s in 308 that run mid length 16" barrels. My son in laws DPMS LR308 is like that and runs like a champ. Though I agree with you guys that I would rather have an 18 or 20" rifle length gas or a +2 length on longer.

Whats the fill capacity on 38grs of powder? That's not a lot of powder in that case. Maybe your getting some pressure spiking?

Edit: Went and answered my own question. At 38grs you're at 73% case fill. Which opens the door to possible pressure spiking. I had this conversation with one of the Berger ballisticians this fall regarding my 300 Norma, and how that empty space effects the pressure curve. He ran some really cool simulations for me on something that seemed to approximate Quickload, but it was Berger's in house program. Not saying it's your problem, but the potential is there.

You are shooting light 120gr bullets, so not much case intrusion. You could easily run that over 43 to 44grs of H4350. Buy a lb of 4350 and shoot it at 42.5grs and see what happens.
Click to expand...

That's true, but .308 pressures are not even close to Creedmoor pressures. In fact, there's many Creedmoor's that have issues running a standard rifle length system. You combine bullets with a long bearing surface with the burn rates of the powders that run them, and pressures spike. There's reasons why JP and some of these other companies have developed the +2 length. I am going to maintain your port pressure is just way too high.
 
#11
The problem is nobody here is testing case pressure or loading to pressure specs. Most people are looking for maximum speed before pressure becomes a problem.

It's the same problems again and again. The problem with dwell time isn't the excess, it's the lack thereof. The gas is blowing back through the tube and unlocking the bolt while it's still under pressure. He needs MORE dwell time. Heavier carriers and buggers can help but only to a point.

My next barrel will be a +2 gas system barrel because that's the source of the issue. It's magnified on a shorter gas system like mid length or carbine. He's going to be fighting that forever. You're better off getting a new barrel set up properly, that or be prepared to download the cartridge to 308 velocity to try and reduce pressure in conjunction with heavier components.
 
#13
^^^ My understanding is that you have 2 things going on here. First, you have the time the bullet is in the barrel before it passes the gas port. The longer the barrel is before the port, the lower the pressure will be in the chamber when the bolt begins to unlock. Second, you have the time the bullet is in the barre after it passes the gas port. The longer the barrel is after the gas port, the more gas that is available to work the action. A rifle length or +2 gas system allows the pressure in the chamber to reduce more than a mid or carbine length system will before the bolt unlocks. AR-10 rifles are inherently harder on brass than bolt rifles, and signs that we would ascribe to high pressure loads in bolt rifles can easily be caused by early unlocking in gas guns.

As this rifle has an adjustable gas block, I think I would turn the gas all the way off and then retest the loads. At least then you would not be fighting the gas system to look for pressure signs in your loads. If you are still getting pressure signs then you know if is the load. If you are not, you can troubleshoot the gas system...
 
#14
Update: loaded up some H4350 from 40.5 up to 43.5 and shot them today. Primers were still cratering at all load levels but not flattening as bad as the Varget loads. Wipe marks are not as deep as the Varget either but still there. Early shots I did shut off the gas completely but didn’t seem to change anything as I opened it back up. Also as a side I loaded up similar loads in some Starline small primer brass but I get bad primer flows on even the lighter loads (silver primer far right).

I guess I will have to reach out to Faxon and see what they say about this.
 

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#16
I wouldn't run a 6.​​​​​​5CM gas gun without the following; a high pressure bolt with the small diameter firing pin hole, an RLGS or preferably RLGS +2 although my RLGS runs very well I would still go longer if given the chance, an adjustable gas block, a heavy buffer and heavy spring.

This ^^^ stuff is mandatory and we see thread after thread from others who don't conform to idea of needing the above parts list and then have problems. On the flip side we see consistent reports from those who have the correct parts and have great reliability.

This information has been available for years on ARF and various other forums granted most of the old threads got deleted with Scout the info is still available. So why are we still building guns without the necessary equipment folks?

OP I'm not sure what the firing pin diameter is on the Faxon bolt I skimmed though their webpage and didn't see it listed. And Faxon it should be listed if you're selling it as a 6.5cm bolt. Why manufactures would sell non HP bolts for 6.5cm at all is aggravating especially when they're coupling that bolt with a mid length gas and I've seen some who are doing just that. I can't tell from your pic if your powder change has resolved the swipes but if it has then you've resolved the short lock time issue. A high pressure bolt should resolve the pressure issues on the primer. The JP HP bolt is a great part very high quality, I've no experience with the Areo bolt but it's less expensive and all their bolts except the NIB bolt are HP. The next question is depending on the specs of the Faxon, can you run a different bolt in your Faxon BCG or do you need an entire BCG? I can't answer that maybe someone else will chime in.
 
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#17
Birddog6424 said:
I'm not so sure that's true.

SAAMI lists max pressure on both cartridges at 62,000PSI. In cruising through my reloading manual (yes, I still like the books) pretty much every load for both cartridges falls in between 58k and 61k PSI. They are pretty neck and neck for pressure. They each have a lot of loads bumping up against max pressure throughout a variety of bullet weights.

I think the 308 AR10 was finicky long before the .260 and 6.5's came along. I think manufacturers have progressed in figuring out the cause of it.

I'm not trying to discount that the barrel and it's gas length aren't the problem. At the end of the day you could very well be right. Either way you slice there is some troubleshooting to be done by the OP to eliminate as many possibilities as he can before giving up on the barrel.
Click to expand...

Well dude, I'm not going to go down that road, because I wasn't addressing chamber pressure, though I should've specified. Pressure at the port is more on the 6.5. It's why the OP is still having problems and will continue to have problems.

ETA - Also, as Roamin has stated, you need to have the right equipment if you're going to go down this road. There's a reason why the gas over-pressure accessory market has exploded in the past 5 years and it's not because 7.62 has suddenly started causing overgassing issues.
 
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#20
I havent read the entire thread, but i was speaking for a manufacturer the other day, and he indicated that they’’ve seen a number of blow outs in the gas tubes for AR10s in 6.5CM.

I asked why that was, and he indicated that the ammo manufacturers have proprietary powder blends to have the rounds to 2700FPS.

He went on to say that while H4350worked great in bolt guns, it was blowing gas tubes in gas guns.

Bolt gun load: Hornday 140 BTHP 41.4gr H4350

AR10: WW760 was the powder, didnt catch the load; He indicated not making it to 2700FPS, more like 2650

Humph.

FWIW OP: My Larue upper came with a small hole firing pin. Looks basically the same as my 308 bolt, but a smaller firing pin hole.
 
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#21
Bigshooter42 said:
Hi guys,

Need to consult the knowledge base here. I'm not new to gas guns but new to the AR10 platform and am having some pressure issues with my 6.5C build and hand loads. Looking for some direction on how to correct this so I can get to shooting.

Build Detail:
Faxon Firearms 16" 6.5 Creedmoor (mid length gas)
I purchased the factory headspaced matching bolt from Faxon as well
Superlative arms bleed off gas block
Springco orange buffer spring with 5.3oz buffer

Load:
Varget (I know this is not ideal powder but I am converting from 308 to 6.5 and have a lot of Varget to use)
Started my load at Hodgdon's recommendation of 36.0 and worked up to about 38.5
Hornady brass with 120gr ELD match bullets and Winchester primers

Issue:
Even at the lower end of the charging scale I am getting cratering primers and bad ejector swipes. I have turned down the gas block to only use enough to cycle the action. I had similar results with Hornady factory ammo (both the 120 ELDs and 129 SSTs). I recently added the springco and heavier buffer thinking this would correct the issue. It did get better because before that change my primers were flowing badly, but they are still cratered and even blew one out at the 38.5 load.

Any recommendations on where to go from here would be much appreciated. I am not sure if it's my barrel, the powder choice, or lack of a heavy enough buffer to get this working. I do have some H4350 but I was going to use that for my bolt gun loads.

Thanks!
Click to expand...

It's the barrel. Way overgassed.

Playing around with powder types might help. Like the lowest IMR4350 or H4350 load you can come up with might work better than that varget one.

Not unsafe but your brass life will suffer.
 
#23
Birddog6424 said:
If it's causing the same problems with the gas turned off then it's not a gas issue. Barrel and gas length are out of the equation, currently.

I'm beginning to lean towards a problem with the cut of your chamber/barrel. Possibly the pin hole diameter of the bolt, although there are still other problems. You shouldn't be seeing theses issues in a bolt that isn't even moving with a light 120gr bullet. I shoot 42.7grs H4350 behind a 140gr RDF in my 6.5.
Click to expand...

Yeah at this point I am going to see if I can send the barrel and BCG back to Faxon to check specs. Hopefully they can send a replacement or refund if it is in fact the barrel or bolt.
 
#24
GBMaryland said:
I havent read the entire thread, but i was speaking for a manufacturer the other day, and he indicated that they’’ve seen a number of blow outs in the gas tubes for AR10s in 6.5CM.

I asked why that was, and he indicated that the ammo manufacturers have proprietary powder blends to have the rounds to 2700FPS.

He went on to say that while H4350worked great in bolt guns, it was blowing gas tubes in gas guns.

Bolt gun load: Hornday 140 BTHP 41.4gr H4350

AR10: WW760 was the powder, didnt catch the load; He indicated not making it to 2700FPS, more like 2650

Humph.

FWIW OP: My Larue upper came with a small hole firing pin. Looks basically the same as my 308 bolt, but a smaller firing pin hole.
Click to expand...

LOL! No shit, huh? Someone should tell Faxon that...putting a mid-length gas system on anything 6/6.5 is begging for problems. I don't know shit about barrel manufacturing but even I know that running a 16" mid-length gas system on a high port-pressured round is going to cause the end user some consternation for certain. As I stated before, you have guys running rifle-length, and +2 systems with overgassing problems.
 
#25
Update.

It took Faxon a while to get a look at everything given the holidays and Shot Show. They determined that eveything was to spec and agreed to refund the cost of the BCG that I purchased from them so I could try out a JP bolt (even returned the BCG to me). I have since purchased the JP high pressure bolt and drum roll........I am pleased to report zero primer issues after some testing at the range today.

Seems like JP is the only way to go for a shorty 6.5 gas gun.

Thank you JP and all that chimed in to this thread along the way.
 
#27
Bigshooter42 said:
Update.

It took Faxon a while to get a look at everything given the holidays and Shot Show. They determined that eveything was to spec and agreed to refund the cost of the BCG that I purchased from them so I could try out a JP bolt (even returned the BCG to me). I have since purchased the JP high pressure bolt and drum roll........I am pleased to report zero primer issues after some testing at the range today.

Seems like JP is the only way to go for a shorty 6.5 gas gun.

Thank you JP and all that chimed in to this thread along the way.
Click to expand...


Rubber City Armory makes a very high quality High Pressure bolt as well. Lifetime Warranty and cheaper. I own and use many of both, they both work great.
 
#29
padom said:
Rubber City Armory makes a very high quality High Pressure bolt as well. Lifetime Warranty and cheaper. I own and use many of both, they both work great.
Click to expand...
I also went RCA for my 18” RLGS BA barrel and just after a quick check with go-no go gauges everything went smoothly since then. No more primer issues whatsoever I will finish this off with JP SCS but very happy now.
 
#30
Rockymtnshooter said:
Padom, Are the rubber city bolt and the jp Interchangeable as far as head spacing goes ? I Have a barrel on order that is head spaced of a jp bolt, but the rubber city price is inviting.
Click to expand...

That I do not know, I always say check with a headspace gauge. I always have my barrels headspaced to my bolts and they live with that barrel so I dont know.
 
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