Over-gassing/Timing Issues

[notagrendel]

Thinking cap time. This maybe a post for the reloading forums.

I am having issues with belting in my 6mm ARC handloads and I am pretty sure it is timing related, but not sure completely. The combo is a 108gr Berger Elite hunter on top of 26.5gr of Varget using a Federal GM205M primer. Starline Once fired 6.5 Grendel formed at 1.182" HS (B350). COAL is 2.280". Velocity is at 2445 fps.

Here is my current built:
Anderson RF-85 upper
Proof Research 18" barrel in 6mm ARC
Rifle +1 gas system w/ SA AGB
JP Enhanced 6.5 Grendel bolt
Brownell's (I think) phosphate carrier
A2 rifle buffer spring and tube
A2 rifle buffer: replaced the aluminum spacers with steel weights from a standard carbine buffer (estimated total weight around 7.5 oz.). I will verify the weight tonight.
ASC 6mm ARC 5-rd. magazine.
Custom 6mm silencer.

When utilizing this load with 6 clicks from closed of the AGB, I did not have any belting*, the action cycled, but the bolt did not lock back. 7 clicks the bolt locked back and there was a slight noticeable belt that I can barely feel with my fingernail, but could not see with a naked eye. 8 clicks the bolt locked back and I noticed the belting occurred maybe a few thousandths up the case.  At this time I did not inspect the cases, for whatever reason.

Two days later I utilized that same load with 20 rounds with 7 clicks from closed on the the AGB. Half of them had belting on them.

Here are the big questions: Is my extra heavy buffer causing timing issues? Is the AGB opened up too much causing the case to begin extraction before pressures drop in order to compensate for the bolt not locking back?

*The way I measure belting is measured in feel and measurement. Belting seems to begin occurring at the webbing measuring 0.4405-0.441"

[StoneHendge]

I don't think your issue is timing., Especially with an 18er with RLGS+1.  There's no way the case is going to move when it's under pressure - it's gripping the chamber walls a lot harder than the bolt can pull it.

Timing is usually manifested with extractor marks on the case head - the bolt tries to rotate while the brass is still under pressure and the metal in the case head gives since the case under pressure won't budge. Back when I played around with 22 Nosler, timing was so bad the rims on the stupid rebated case heads would literally bend from getting yanked so hard.

Your likely near your limit. I use 26.8 grains of Varget in converted Starline 6.5G with GM205Ms with a 105 RDF at 2450 out of my 16" and I know I'm just shy of max. I have an MLGS and get an occasional case head smear, but timing issues are more manageable than I thought they'd be.

With a AGB, I would try and go as light as possible with the buffer - you can always dial the gas down to compensate. The result is slightly less pulse/felt recoil but the biggest benefit I find is the reduced slam on return to battery, which throws your sight picture off as much as recoil, if not more.

[r.tenorio671]

...you can also try firing one of those rounds with your gas closed off completely and see if you get that "belt" condition on the brass. I would also recommend you use a standard buffer to start out with....always a good idea to start with standard "baseline" parts so you can determine what, if anything, needs to be adjusted and which direction that adjustement needs to go.

[popgun]

I'm by no means an expert ... but, could it be you have too much headspace? My fired brass measures 1.190-1.191 and resize to 1.188 or a half thou under that. Your around .008-.009" of headspace if your chamber is close to what I've seen common for dimensions. That would be working the brass pretty hard and don't think it would last as long as most before signs of case separation. My novice opinion should be taken with a chunk of rock salt.

[popgun]

Thinking cap time. This maybe a post for the reloading forums.

I am having issues with belting in my 6mm ARC handloads and I am pretty sure it is timing related, but not sure completely. The combo is a 108gr Berger Elite hunter on top of 26.5gr of Varget using a Federal GM205M primer. Starline Once fired 6.5 Grendel formed at 1.182" HS (B350). COAL is 2.280". Velocity is at 2445 fps.

Here is my current built:
Anderson RF-85 upper
Proof Research 18" barrel in 6mm ARC
Rifle +1 gas system w/ SA AGB
JP Enhanced 6.5 Grendel bolt
Brownell's (I think) phosphate carrier
A2 rifle buffer spring and tube
A2 rifle buffer: replaced the aluminum spacers with steel weights from a standard carbine buffer (estimated total weight around 7.5 oz.). I will verify the weight tonight.
ASC 6mm ARC 5-rd. magazine.
Custom 6mm silencer.

When utilizing this load with 6 clicks from closed of the AGB, I did not have any belting*, the action cycled, but the bolt did not lock back. 7 clicks the bolt locked back and there was a slight noticeable belt that I can barely feel with my fingernail, but could not see with a naked eye. 8 clicks the bolt locked back and I noticed the belting occurred maybe a few thousandths up the case.  At this time I did not inspect the cases, for whatever reason.

Two days later I utilized that same load with 20 rounds with 7 clicks from closed on the the AGB. Half of them had belting on them.

Here are the big questions: Is my extra heavy buffer causing timing issues? Is the AGB opened up too much causing the case to begin extraction before pressures drop in order to compensate for the bolt not locking back?

*The way I measure belting is measured in feel and measurement. Belting seems to begin occurring at the webbing measuring 0.4405-0.441"

Is this what you refer to as belting?

[Lemonaid]

The pics remind me of questions some have had with cases after sizing, concerned about case head separation. It looks to me you have an bit of frosting from your chamber that stops at the case head as it is mostly solid brass. On highly over pressure brass, the case head can expand but that should be easy to check. Measure case head on unfired brass and compare. I think you would see some obvious pressures signs, ejector marks, extractor marks and primer flattening if were hot enough to expand the case head.

[popgun]

The pics remind me of questions some have had with cases after sizing, concerned about case head separation.  It looks to me you have an bit of frosting from your chamber that stops at the case head as it is mostly solid brass.  On highly over pressure brass, the case head can expand but that should be easy to check.  Measure case head on unfired brass and compare.  I think you would see some obvious pressures signs, ejector marks, extractor marks and primer flattening if were hot enough to expand the case head.

 I posted those pictures I found online. I'm not familiar with the term "belting" and posted the pictures to clarify. Sorry for the confusion.
 My thought was if the Starline brass is a bit brittle, shrinking it down .035"+ and headspacing 8 or 9 thousandths might cause work hardening above the webbing.

[notagrendel]

Thinking cap time. This maybe a post for the reloading forums.

I am having issues with belting in my 6mm ARC handloads and I am pretty sure it is timing related, but not sure completely. The combo is a 108gr Berger Elite hunter on top of 26.5gr of Varget using a Federal GM205M primer. Starline Once fired 6.5 Grendel formed at 1.182" HS (B350). COAL is 2.280". Velocity is at 2445 fps.

Here is my current built:
Anderson RF-85 upper
Proof Research 18" barrel in 6mm ARC
Rifle +1 gas system w/ SA AGB
JP Enhanced 6.5 Grendel bolt
Brownell's (I think) phosphate carrier
A2 rifle buffer spring and tube
A2 rifle buffer: replaced the aluminum spacers with steel weights from a standard carbine buffer (estimated total weight around 7.5 oz.). I will verify the weight tonight.
ASC 6mm ARC 5-rd. magazine.
Custom 6mm silencer.

When utilizing this load with 6 clicks from closed of the AGB, I did not have any belting*, the action cycled, but the bolt did not lock back. 7 clicks the bolt locked back and there was a slight noticeable belt that I can barely feel with my fingernail, but could not see with a naked eye. 8 clicks the bolt locked back and I noticed the belting occurred maybe a few thousandths up the case.  At this time I did not inspect the cases, for whatever reason.

Two days later I utilized that same load with 20 rounds with 7 clicks from closed on the the AGB. Half of them had belting on them.

Here are the big questions: Is my extra heavy buffer causing timing issues? Is the AGB opened up too much causing the case to begin extraction before pressures drop in order to compensate for the bolt not locking back?

*The way I measure belting is measured in feel and measurement. Belting seems to begin occurring at the webbing measuring 0.4405-0.441"

Is this what you refer to as belting?

That's what the factory ammo looks like, but my handloads have a rim at the very base of the body, yet pressure signs of primer flattening, ejector marks and extractor marks are more present on the factory than my handloads.

---

...you can also try firing one of those rounds with your gas closed off completely and see if you get that "belt" condition on the brass.  I would also recommend you use a standard buffer to start out with....always a good idea to start with standard "baseline"  parts so you can determine what, if anything, needs to be adjusted and which direction that adjustement needs to go.

I just got back today from the range, and I tested out some loads that I thought weren't belting when I worked them up. They belted with the AGB closed to 1 click past LRBHO.

I'm by no means an expert ... but, could it be you have too much headspace? My fired brass measures 1.190-1.191 and resize to 1.188 or a half thou under that. Your around .008-.009" of headspace if your chamber is close to what I've seen common for dimensions. That would be working the brass pretty hard and don't think it would last as long as most before signs of case separation. My novice opinion should be taken with a chunk of rock salt.

My chamber HS is at 1.1855"-1.186", so 1.181" is only 0.004"

[notagrendel]

I don't think your issue is timing., Especially with an 18er with RLGS+1.  There's no way the case is going to move when it's under pressure - it's gripping the chamber walls a lot harder than the bolt can pull it.

Timing is usually manifested with extractor marks on the case head - the bolt tries to rotate while the brass is still under pressure and the metal in the case head gives since the case under pressure won't budge. Back when I played around with 22 Nosler, timing was so bad the rims on the stupid rebated case heads would literally bend from getting yanked so hard.

Your likely near your limit. I use 26.8 grains of Varget in converted Starline 6.5G with GM205Ms with a 105 RDF at 2450 out of my 16" and I know I'm just shy of max. I have an MLGS and get an occasional case head smear, but timing issues are more manageable than I thought they'd be.

With a AGB, I would try and go as light as possible with the buffer - you can always dial the gas down to compensate. The result is slightly less pulse/felt recoil but the biggest benefit I find is the reduced slam on return to battery, which throws your sight picture off as much as recoil, if not more.

What's your first sign's of pressure? I'm getting more pressure signs on the primers, ejector marks and extractor marks using Hornady 108gr Match (these don't belt) than the belting handloads.

[r.tenorio671]

Quote: "I just got back today from the range, and I tested out some loads that I thought weren't belting when I worked them up. They belted with the AGB closed to 1 click past LRBHO."

...by having your AGB set to "1 click past LRBHO" you are defeating the purpose of the test to determine if you are getting an early unlock of your bolt due to overpressure from a hot charge weight. If the gas is turned off completely to prevent the BCG from cycling then the bolt will not attempt to unlock. An early unlock can occur with overpressure rounds causing the casing to start extracting while pressure is still high enough that the resulting unsupported portion of the casing web "over expands" due excess pressure still present. Typically, the resulting bulge will primarily be on one side of the casing, but could also be completely around the full circumference of the casing. One thing that wasn't mentioned is what dies are you using? In the last picture posted those lines delineating the "band" on the web are typical when using small based sizing dies that squeeze the casing down to minimum SAAMI spec (commonly used for ammo to be used in semi-autos to ensure chambering success across differing brands of barrels). Your load of 26.5 of Varget is almost at Hornady's max of 26.7 and over Hodgdon's 25.8C (compressed for 107SMK) for Varget. The symptoms you have related are common to an overcharged load, i.e., case web expansion, cycling issues, buffer/gas block adjustments, etc. These are why it's recommended to use the standard buffer & buffer spring initially to get baseline data before making changes to anything.

[StoneHendge]

I don't think your issue is timing., Especially with an 18er with RLGS+1.  There's no way the case is going to move when it's under pressure - it's gripping the chamber walls a lot harder than the bolt can pull it.

Timing is usually manifested with extractor marks on the case head - the bolt tries to rotate while the brass is still under pressure and the metal in the case head gives since the case under pressure won't budge. Back when I played around with 22 Nosler, timing was so bad the rims on the stupid rebated case heads would literally bend from getting yanked so hard.

Your likely near your limit. I use 26.8 grains of Varget in converted Starline 6.5G with GM205Ms with a 105 RDF at 2450 out of my 16" and I know I'm just shy of max. I have an MLGS and get an occasional case head smear, but timing issues are more manageable than I thought they'd be.

With a AGB, I would try and go as light as possible with the buffer - you can always dial the gas down to compensate. The result is slightly less pulse/felt recoil but the biggest benefit I find is the reduced slam on return to battery, which throws your sight picture off as much as recoil, if not more.

What's your first sign's of pressure? I'm getting more pressure signs on the primers, ejector marks and extractor marks using Hornady 108gr Match (these don't belt) than the belting handloads.

That's the hard part with calibers that can have timing issues - sometimes ejector/extractor marks are pressure and sometimes they are timing. When they are from pressure, they usually seem to show up before flattened primers. I shoot a lot of .223 and 6x45 which don't suffer from timing (other than my 16" 5.56 with a carbine gas system), and they always seem to get smears before primers flatten as I get around published maxes. It wouldn't surprise me if you get pressure signs from Hornady Match - it's loaded at max and if you have a tight chamber and/or it's a warm day, it might be too much. I've heard plenty of stories of guys here in Phoenix who pierce primers with 5.56 ball ammo in 100+ degree days.

The easiest way to check is to turn off the gas for a few rounds. That's how I always get somewhat comfortable with ejector marks. And I mean somewhat, because even if you can alleviate them, the timing issue is still there and the extractor is still getting somewhat abused.