Bolt Lug Deformation - Proof SS 6ARC Barrel

[AMwood]

Recently put together a 6 ARC gasser using a 22” SS Proof barrel in a Seekins IRMT upper with a new Maxim bolt. It shoots great however, after running a mix of factory 108 Hornady Match and running a load work up with 26.6 gr of 8208 XBR under an 87 gr vmax I found the deformation on the back of the bolt lugs:

This is after approximately 100 rounds. I’m not seeing pressure signs on the brass. Ejection using a suppressor with adjustable gas block, rifle buffer and JP polished/tuned spring is at 3 o’clock. Velocity gave an average of 2823 fps with an sd of 5 so it’s been very consistent. I double checked I wasn’t seating in the lands causing a pressure spike.

Any ideas what could be causing this? Should I run it as is? I also tried a different used no name MPI bolt to see if I got a lemon but came back with almost identical deformation on it after only 30 rounds.

 

[msgriff]

That looks significant enough to affect headspace. You might already be beyond no-go, maybe even max.

 

[Ebar48]

Too much PX, I don’t have access to my quickload computer now but I would have someone run the reloads you are using, bolt thrust on the Grendel/arc case is a known issue so you can’t push it past published max too much or you get the issues you are seeing. Hodgdon shows 27.2 as the max load with an 80gner, so bumping up to the 87, you are probably a bit above max. Granted where I’m at this is the only data I have available to me but seeing as how you had the same issue with two bolts, you need a new bolt and to back down your load IMO.

 

[AMwood]

Too much PX, I don’t have access to my quickload computer now but I would have someone run the reloads you are using, bolt thrust on the Grendel/arc case is a known issue so you can’t push it past published max too much or you get the issues you are seeing

That’s the thing, Hornady gas gun load data maxes at 27.1. Every barrel is different but I’m a half grain under so that’s why I’m struggling to accept it’s pressure. This was shot during 60-65 degree weather so it can’t be a temp sensitivity thing.confused to say the least.

 

[Ebar48]

At the end of the day the PX in the chamber is enough that is it pushing back on the bolt face hard enough to physically deform metal, you have lug setback annd it’s opening up your headspace. And as the previous poster said you probably are at max or over headspace now with that bolt. Biggest thing to understand is lot to lot consistency, not all powder is the same lot to lot. What is the published barrel length and velocity in the hornady book? xbr is on the faster side for that weight class, get a new bolt and run a different powder, I would recommend lvr.

 

[Greg Langelius *]

Has anyone considered a heat treating issue?

Greg

 

[Ebar48]

Has anyone considered a heat treating issue?

Greg

The fact that he had it happen to two bolts both in less than a 100 rounds makes it unlikely it was a one off heat treat issue

 

[AMwood]

6ARC go/no go gauges ordered along with a new bolt via Brownells. We’ll see what comes after I receive them.

 

[AMwood]

Maybe someone with more experience can answer this for me, but if it is indeed the powder in the load would you expect to see obvious pressure signs on the brass?

The brass below is dirty from my can and shows some light marks from the extractor. In my experience this isn’t out of the norm for a gas gun though. The once fired factory brass appears nearly identical.

 

[Ebar48]

You most likely won’t see “traditional” PX signs on the brass as the max PX in an AR is like 10k less psi then a bolt gun, but the fact you are bushing back the lugs is in fact your absolute pressure sign.

 

[newguy2k3v2]

The issue is bolt thrust. The larger case head on the grendel case means more bolt thrust for the same chamber pressure as say 5.56. That’s why grendel and arc loads list a lower “max” pressure for gas guns. Pressure that overloads the bolt in the grendel case isn’t enough to show typical signs on the brass.

 

[msgriff]

Not a current solution, but this design would certainly help with the 6ARC bolt thrust/pressure issues.

 

[TheOE800]

@Constructor needs to find a box of bolts and extensions…

 

[AMwood]

Outside of it being a powder related issue like @Ebar48 mentioned above what else could lead to overpressure similar to what I’m seeing? Not ruling out it being a load problem, but at $50+ depending on the bolt I’d like to entertain other possibilities. Could a tight chamber present the bolt symptoms I’m seeing?

 

[Kinked_Chrome]

Even though port ejection placment seems ok, id still try turning the gas down. For what its worth, my 6 arc bolt got eaten up pretty fast also, and i had the gas set so that itd barely hold the bolt open. It was a 24in rifle gas length ballistic advantage barrel. If i had to do it all over again, id get at least a plus 2 system for the same length.

When i finally ditched that bolt and barrel, it was at 1500rds or so and my fired brass was like .020in over the no go gauge. I had to size the brass to the chamber basically every 200rds or so otherwise the headspace would be to much. The bolt looked like shit, but never quit working.

 

[MarshallDodge]

Sidetrack: I never understood why someone thought it was a good idea to take all that material off a bolt rather and not correct the extension at the same time. If you are changing a barrel anyway...

 

[msgriff]

Even though port ejection placment seems ok, id still try turning the gas down. For what its worth, my 6 arc bolt got eaten up pretty fast also, and i had the gas set so that itd barely hold the bolt open. It was a 24in rifle gas length ballistic advantage barrel. If i had to do it all over again, id get at least a plus 2 system for the same length.

When i finally ditched that bolt and barrel, it was at 1500rds or so and my fired brass was like .020in over the no go gauge. I had to size the brass to the chamber basically every 200rds or so otherwise the headspace would be to much. The bolt looked like shit, but never quit working.

I hope you misplaced a decimal on your dimension. 6ARC chamber 1.1901 min - 1.2001 max.

 

[msgriff]

Outside of it being a powder related issue like @Ebar48 mentioned above what else could lead to overpressure similar to what I’m seeing? Not ruling out it being a load problem, but at $50+ depending on the bolt I’d like to entertain other possibilities. Could a tight chamber present the bolt symptoms I’m seeing?

What length is your gas system? It would also be a good idea to inspect the inside of the barrel extension lugs.

 

[TheOfficeT-Rex]

Quick GRT puts it at +/-47K PSI.

Hows the lockup of the bolt (and the new bolt) in the extension? Is the bolt getting a 'running start' at the lugs?

 

[Kinked_Chrome]

I hope you misplaced a decimal on your dimension. 6ARC chamber 1.1901 min - 1.2001 max.

No, misplacement of the decimal. 20 thousandths of an inch over the no go. I guess technically it wasnt a 6 arc any more. It was a 6 ARC+ or something. More room for powder is what i say.

 

[ma smith]

Maybe someone with more experience can answer this for me, but if it is indeed the powder in the load would you expect to see obvious pressure signs on the brass?

Maybe someone can explain away the pressure signs from you ejector and extractor. Or maybe I'm just seeing things.

 

[Constructor]

Nitride bolt or Black Oxide? I've been telling people for several years that nitride treating bolts or barrel extensions is not a good idea. It softens the bolt and extension lugs and over a period of time they will compress leading to excessive headspace.
Maxim description does not say if they are Nitride treated or Black Oxide.

 

[TonyTheTiger]

Maybe someone can explain away the pressure signs from you ejector and extractor. Or maybe I'm just seeing things.
View attachment 8137912

It says Hornady. There's your explanation.

 

[AMwood]

What length is your gas system? It would also be a good idea to inspect the inside of the barrel extension lugs.

It’s a rifle +1. Part of Proof’s “camgas” spec. I would have liked a +2 but it wasn’t an option, unfortunately. It’s tough to get a good look at the inside of the extension lugs. I don’t own a bore scope.

 

[AMwood]

Quick GRT puts it at +/-47K PSI.

Hows the lockup of the bolt (and the new bolt) in the extension? Is the bolt getting a 'running start' at the lugs?

Interesting, I’m of the mind the load would be hard pressed to be the issue without there being another variable at play. I’ve used the same keg of 8208 on a Grendel and 223 rounds without seeing anything off putting.

Haven’t received the new bolt yet. The lock up was good on the old one though. I did have some issues with alignment between the bolt and extension but solved that by locking the barrel in my barrel vice when torquing the receiver nut down. Once aligned it went into battery like any other AR I own.

 

[AMwood]

Nitride bolt or Black Oxide? I've been telling people for several years that nitride treating bolts or barrel extensions is not a good idea. It softens the bolt and extension lugs and over a period of time they will compress leading to excessive headspace.
Maxim description does not say if they are Nitride treated or Black Oxide.

I honestly don’t know. The shot peening gives the bolt an almost smooth phosphate appearance.

 

[Precision Underground]

That’s pretty wild. Am I seeing it right that there are like .01-.02 deep indentions in the bolt? I would think if you’re over pressure so much that it stamps the lugs, you’d see other signs. What brand bolt is it?

Could there be slop between the extension and the lugs so it’s getting a running start when it’s fired? I feel like it would take an incredible amount of pressure to stamp the lugs like that if they were seated properly. @Constructor may have some ideas.

 

[whatsupdoc]

Cant tell from the picture but it may not be what you think, I have seen the boltlugs get shaved
from the barrel extension when there is too much slop between the barrel locating pin and the alignment slot
in the upper causing a barrel misalignment combined with sharp edges on the extension.

Clean inside the barrel extension with a qtip and look for steel shavings and check when the bolt
is traveling past the extension if the bolt lugs are centered between the extension lugs.

I would contact the barrel mfg and the bolt mfg and pick their brains.

 

[Constructor]

The fact that he had it happen to two bolts both in less than a 100 rounds makes it unlikely it was a one off heat treat issue

T

That’s pretty wild. Am I seeing it right that there are like .01-.02 deep indentions in the bolt? I would think if you’re over pressure so much that it stamps the lugs, you’d see other signs. What brand bolt is it?

Could there be slop between the extension and the lugs so it’s getting a running start when it’s fired? I feel like it would take an incredible amount of pressure to stamp the lugs like that if they were seated properly. @Constructor may have some ideas.

Around 2007-08 there weren't many manufacturers nitride/melonite treating barrels, LWRC was likely the first and I wasn't far behind. The places like L&M and Bodycote were telling everyone nitride treating barrels with the barrel extensions on the barrels was not a problem, it would not effect the assembly. In the first 100 barrels I found out that was wrong in more than a few ways. First I could almost remove the extensions with my bare hands. Second the hardness of the extensions dropped about 10 points. Video below shows me removing the extensions with just a blip of the impact when the torque should be 150ftlbs. I removed all of the extensions and replaced them with properly heat treated extensions, you can see the silver ones behind.
At that point in time we were testing the 6.8 heavily. It didn't take long to see the same thing the OP posted but the compression was on the extension lugs because they were Nitride treated. The bolts I was machining were gas carburized to 60 Rockwell and then parkerized/mag phos. so they did not compress.
The third thing was barrels that are treated with the extensions on had the salt residue in the threads between the extension and barrel threads, if they had been left alone without removing the extension and cleaning the threads I'm sure the salt would have corroded the threads away. I guess I'm one of those guys that never wear hearing protection or rubber gloves... When you first get your Nitride barrels clean them off well wearing rubber gloves.
Warning about salt bath nitriding-

Problems with salt bath nitriding​

As we’ve alluded, salt bath nitriding can lead to some problems in terms of both safety and cost:

• Caustic cyanide solutions put operators at risk of serious injury if molten salt contacts their skin.
• Parts with unsealed seams or complex geometries can trap the salt solution even after rigorous rinsing. That puts workers at risk for additional caustic exposure if the salt shakes loose later and can increase the risk of corrosion damage to parts.
• Molten salt baths cannot be turned off. The salt must remain molten, so baths must stay heated even when they’re not in use.

 

[Ernest 5.56]

Interesting, I’m of the mind the load would be hard pressed to be the issue without there being another variable at play. I’ve used the same keg of 8208 on a Grendel and 223 rounds without seeing anything off putting.

Haven’t received the new bolt yet. The lock up was good on the old one though. I did have some issues with alignment between the bolt and extension but solved that by locking the barrel in my barrel vice when torquing the receiver nut down. Once aligned it went into battery like any other AR I own.

Can you please elaborate about the alignment ? Was this during the test fit/ build?

 

[AMwood]

That’s pretty wild. Am I seeing it right that there are like .01-.02 deep indentions in the bolt? I would think if you’re over pressure so much that it stamps the lugs, you’d see other signs. What brand bolt is it?

Could there be slop between the extension and the lugs so it’s getting a running start when it’s fired? I feel like it would take an incredible amount of pressure to stamp the lugs like that if they were seated properly. @Constructor may have some ideas.

They are indentations. I don’t have a great way to measure the depth of the indent but a caliper was giving me anywhere from .007-.009. It’s a Maxim bolt (not trying to put them on trial by any means here since I think there are other issues at play and it occurred with two completely different bolts).

There is a bit of wiggle between the back of the bolt lugs and the extension but it’s small. No idea what the clearance should be for parts that lock and unlock in an action like that so it might be in spec for all I know.

 

[AMwood]

Can you please elaborate about the alignment ? Was this during the test fit/ build?

During the build. I’m using a Seekins IRMT upper which has a degree of of clearance in the index pin channel. When torquing the barrel nut on, the receiver would rotate very slightly causing the lugs of the extension to be off by maybe a degree or two. This would cause the bolt lugs to contact the extension lugs during cycling. Not enough to bind or completely stop the action but to the point that wear light marks were evident. Locking everything down prior to torquing fixed that problem.

 

[AMwood]

T

Around 2007-08 there weren't many manufacturers nitride/melonite treating barrels, LWRC was likely the first and I wasn't far behind. The places like L&M and Bodycote were telling everyone nitride treating barrels with the barrel extensions on the barrels was not a problem, it would not effect the assembly. In the first 100 barrels I found out that was wrong in more than a few ways. First I could almost remove the extensions with my bare hands. Second the hardness of the extensions dropped about 10 points. Video below shows me removing the extensions with just a blip of the impact when the torque should be 150ftlbs. I removed all of the extensions and replaced them with properly heat treated extensions, you can see the silver ones behind.
At that point in time we were testing the 6.8 heavily. It didn't take long to see the same thing the OP posted but the compression was on the extension lugs because they were Nitride treated. The bolts I was machining were gas carburized to 60 Rockwell and then parkerized/mag phos. so they did not compress.
The third thing was barrels that are treated with the extensions on had the salt residue in the threads between the extension and barrel threads, if they had been left alone without removing the extension and cleaning the threads I'm sure the salt would have corroded the threads away. I guess I'm one of those guys that never wear hearing protection or rubber gloves... When you first get your Nitride barrels clean them off well wearing rubber gloves.
Warning about salt bath nitriding-

Problems with salt bath nitriding​

As we’ve alluded, salt bath nitriding can lead to some problems in terms of both safety and cost:

• Caustic cyanide solutions put operators at risk of serious injury if molten salt contacts their skin.
• Parts with unsealed seams or complex geometries can trap the salt solution even after rigorous rinsing. That puts workers at risk for additional caustic exposure if the salt shakes loose later and can increase the risk of corrosion damage to parts.
• Molten salt baths cannot be turned off. The salt must remain molten, so baths must stay heated even when they’re not in use.

Your previous posts came to mind when I encountered the problem. Proof’s extension looks to be stainless and harder than the bolt given the indentations I’m seeing.

I actually looked for a non-nitrided bolt but only came up with some no name nickel boron options. Even if the bolt hardness is suspect, on the second used bolt I had laying around that exhibited the same deformation I actually found two of the lugs were cracked from the 30 or so rounds I used it for to verify if the original bolt was a lemon. One snapped fairly easily when put under some stress from a pair of pliers. Something doesn’t seem to be adding up.

 

[TheOE800]

I believe SOLGW makes a 6.5G/6mm bolt that is phosphated and C158, no fancy shmancy improvements advertised.

 

[Ernest 5.56]

JP EnhancedBolt™ Assembly Caliber: 6.5 Grendel Finish: Black DLC

 

[AMwood]

JP EnhancedBolt™ Assembly Caliber: 6.5 Grendel Finish: Black DLC

Unobtanium. Out of stock everywhere I looked. Thanks for the link though.

 

[JS8588]

Unobtanium. Out of stock everywhere I looked. Thanks for the link though.

6.5 Grendel Type 2 Bolt With 0.136 Bolt Face in DLC

The Sharps Xtreme Performance Bolt (XPB) in DLC (diamond Like Carbon), is our patented Relia-Bolt™ finished in DLC (Diamond Like Carbon) coating.Patented bolt technologyThe redesigned lugs of the Relia-Bolt™ virtually eliminate the most common MSR malfunction; the bolt to barrel extension jam. Have

www.srcarms.com

 

[Tx_Aggie]

Another option that's phosphated:

6.5 Grendel Bolt Assembly | Alexander Arms

www.alexanderarms.com

 

[357Max]

@AMwood

If it'd been 1 bolt I'd say soft heat treat, but with 2 x & different bolt manufacturers that's out the window.

Spitballing a possible cause. How smooth is the chamber finished in that barrel? Also do you get the chamber portion of the barrel clean/de oiled when you clean it?

I've had 1 bolt act very similar to your Maxim & it was a Toolcraft nitride 308. Sent it to them for inspection and they said it was soft & replaced it. I replaced the barrel so don't know if that was in fact the cause.

My 6 ARC Rainier set up did the exact opposite of yours using the same Maxim bolt.

Bolt Won, extension lost.

Head space when new & after about 260 rounds. The No Go guage in second pic has several pc's of scotch tape on it.

Here is the bolt. It does show a little wear, but no displaced metal and can't feel anything with fingernail or a pick. It's still trash!
I didn't catch this issue till head space had grown +.015 My reloading process is measure fired brass on guage pictured above & then bump shoulders back .002. I had no idea it was growing until I tried a piece of brass prepped for this barrel in a new Craddock with JP bolt. Wouldn't close WTF, so I started checking. The reason I explained this last part was so it would be clear that ammo fired never had more than .002 headspace, or in other words even when it started growing it wasn't like the case was getting a .005 or .010 running start. Always had .002 brass head space despite the growth.

Here is what I found in the Tin extension. First the lug the sits behind the extractor i.e. as new.
I'm assuming they missed the hardness spec on the extension. Rainier agreed, but I have doubts they actually tested it.

FWIW I'll be installing the Sharps T7 Type 2 Grendel bolt in the replacement Rainier barrel. JP's are as you said oos everywhere I looked.

 

[AMwood]

@AMwood

If it'd been 1 bolt I'd say soft heat treat, but with 2 x & different bolt manufacturers that's out the window.

Spitballing a possible cause. How smooth is the chamber finished in that barrel? Also do you get the chamber portion of the barrel clean/de oiled when you clean it?

I've had 1 bolt act very similar to your Maxim & it was a Toolcraft nitride 308. Sent it to them for inspection and they said it was soft & replaced it. I replaced the barrel so don't know if that was in fact the cause.

My 6 ARC Rainier set up did the exact opposite of yours using the same Maxim bolt.

Bolt Won, extension lost.

Head space when new & after about 260 rounds. The No Go guage in second pic has several pc's of scotch tape on it.
View attachment 8138818
View attachment 8138819

Here is the bolt. It does show a little wear, but no displaced metal and can't feel anything with fingernail or a pick. It's still trash!
I didn't catch this issue till head space had grown +.015 My reloading process is measure fired brass on guage pictured above & then bump shoulders back .002. I had no idea it was growing until I tried a piece of brass prepped for this barrel in a new Craddock with JP bolt. Wouldn't close WTF, so I started checking. The reason I explained this last part was so it would be clear that ammo fired never had more than .002 headspace, or in other words even when it started growing it wasn't like the case was getting a .005 or .010 running start. Always had .002 brass head space despite the growth.

View attachment 8138828

Here is what I found in the Tin extension. First the lug the sits behind the extractor i.e. as new.
I'm assuming they missed the hardness spec on the extension. Rainier agreed, but I have doubts they actually tested it.
View attachment 8138834View attachment 8138835View attachment 8138836View attachment 8138837

FWIW I'll be installing the Sharps T7 Type 2 Grendel bolt in the replacement Rainier barrel. JP's are as you said oos everywhere I looked.

Thanks for the response. Definitely a similar experience to what I’ve encountered. The chamber doesn’t look overly polished, similar to a brushed stainless finish. I usually wipe it out after cleaning with several oversized cotton patches on a brass loop. There doesn’t appear to be any excess solvent or oil left behind from this. After a few rounds the chamber is usually dusted with carbon from my can’s back pressure so any oil looks like it’s blasted out. The chamber is my next area to focus on though.

Interestingly, when I first test fired the barrel the very first round was a Hornady match round that I fired with the gas block completely closed so I could get an accurate measurement from for sizing. I used this piece to get a rough BTO measurement on the 87 grain vmax and found that the fired neck holds the round with a decent amount of tension. It’s not enough that you can’t push or pull the round with your fingers but it also doesn’t fall through like I’ve encountered with others. My hypothesis is that maybe the chamber is on the tight side and once everything is carboned up from the can, it’s that much tighter and causing the over pressure. I’ve sent Proof an email and am waiting to hear back. The customer service rep said they’re at a trade show so they’d be slow to respond.

 

[GreenGO Juan]

It says Hornady. There's your explanation.

Looks a little warm to me

 

[TonyTheTiger]

View attachment 8138965

Looks a little warm to me

View attachment 8138968

Could very well be warm, but in my Hornady experience with 6.5 Grendel, 6mm Grendel, 5.56 and 9mm, all their brass gets beat up by even light loads. I've got a 223 load, 55gr around 2600fps and it still chews up the Hornady a little while any other headstamp comes out looking new.

 

[GreenGO Juan]

Could very well be warm, but in my Hornady experience with 6.5 Grendel, 6mm Grendel, 5.56 and 9mm, all their brass gets beat up by even light loads. I've got a 223 load, 55gr around 2600fps and it still chews up the Hornady a little while any other headstamp comes out looking new.

Definitely on the soft side. Thats one of the reasons its good to use as a gauge for pressure. IMO

Also I just wanted to say there was some talk about sizing .002 to .003 off of a fired case. Thing is, with a semi sometimes the headspace will stretch on extraction, exasperated if the system is over gassed, not giving you an accurate measurement of the chamber.

Either turn the gas completely off and measure off that case or pull the ejector, pull the bolt out of the bolt carrier and manually see what headspace works and what doesn’t.

Not stating that is what’s causing the sever wear here, just things I have noticed in my progress that helped get to reliable reloads.

 

[AMwood]

My faith in the ARC has been shaken. Response from Proof below.

Disclaimer: I’m not pointing fingers. Appears the current bolts aren’t ready for prime time.

If I have to replace a bolt every few hundred rounds and potentially wade through replacing a barrel extension down the road then I might just cut my losses and go with a different caliber. It’s too bad, there’s an awesome range of projectile types and weights in .243 that really made this a tempting idea.

 

[supercorndogs]

Seems fishy. Why aren't the 6mm fat rats breaking bolts. The 6.5 Gredel runs the same bolt face and pressure. Its broken bolts have long been sorted. If its the smaller the bullet, are people using 22 Gredels breaking bolts even faster? I understand the bolts for this size cartridge in the AR are marginal, but breaking every 200 rounds, i feel there is certainly something else wrong, same goes for breaking in a couple thousand.

 

[AMwood]

Seems fishy. Why aren't the 6mm fat rats breaking bolts. The 6.5 Gredel runs the same bolt face and pressure. Its broken bolts have long been sorted. If its the smaller the bullet, are people using 22 Gredels breaking bolts even faster? I understand the bolts for this size cartridge in the AR are marginal, but breaking every 200 rounds, i feel there is certainly something else wrong, same goes for breaking in a couple thousand.

If there is something else going on or out of spec it’s beyond my capabilities.

I received my Clymer go/no go gauges and with a new bolt it passes with the go and fails the no go. Two layers of scotch tape on the back of the go gauge (approx + .004) was enough to make the new bolt fail. Not sure if this makes the chamber fall on the tight side.

Anyways, maybe I’m just lucky and got a problem child of a rifle.

 

[357Max]

If there is something else going on or out of spec it’s beyond my capabilities.

I received my Clymer go/no go gauges and with a new bolt it passes with the go and fails the no go. Two layers of scotch tape on the back of the go gauge (approx + .004) was enough to make the new bolt fail. Not sure if this makes the chamber fall on the tight side.

Anyways, maybe I’m just lucky and got a problem child of a rifle.

Go + .002-.003 is not tight, but within spec.

I’ve got the Sharps bolt on the way to re-assemble the Rainier build. They replaced the barrel I’d pictured above.

I could have just ordered another Maxim bolt, but want to try the Sharps T7 DLC bolt. I’ve got several of their BCG’s and have been very happy with them.

 

[AMwood]

Go + .002-.003 is not tight, but within spec.

I’ve got the Sharps bolt on the way to re-assemble the Rainier build. They replaced the barrel I’d pictured above.

I could have just ordered another Maxim bolt, but want to try the Sharps T7 DLC bolt. I’ve got several of their BCG’s and have been very happy with them.

I’m tempted to look at one too but I have an unfounded worry that a harder bolt material could wear the barrel extension lugs if it turns out to be softer than the bolt lugs. Replacing a bolt is easier than an extension in my mind.

 

[supercorndogs]

The problem could be as simple as the loads people are using are just to hot.

 

[LRRPF52]

My faith in the ARC has been shaken. Response from Proof below.

Disclaimer: I’m not pointing fingers. Appears the current bolts aren’t ready for prime time.
View attachment 8140187

If I have to replace a bolt every few hundred rounds and potentially wade through replacing a barrel extension down the road then I might just cut my losses and go with a different caliber. It’s too bad, there’s an awesome range of projectile types and weights in .243 that really made this a tempting idea.

Extreme pressures? 52,000psi isn’t extreme pressure. 5.56 NATO is loaded way hotter than that.

I just checked 4 of my Grendels, 2 of which have Maxim bolts that look like yours with the rings machined around the bolt body, black high sheen finish.

My 17.6” Grendel (that I built in 2014 and have shot since then) has a Maxim bolt that looks new. Round count is in the thousands with a mix of:

* Hand loads with 31.2gr of CFE223 under the 123gr A-MAX (I used to shoot this at 100rds per session/event.)
* A lot of Precision Firearms 123gr SMK (they came in 50rd boxes loaded in Lapua brass)
* Precision Firearms 120gr Scenar-L (also came in 50rd boxes)
* Hornady 123gr SST factory ammo
* Cases of 123gr Hornady America Gunner that were sold in the 200rd cans at Sportsman’s (I loaded up my shopping cart with that ammo at $139.99/200rds back when it was sold for that, had my little boy get out of his seat so I could max the cart out on a Memorial Day sale)
* 107gr SMK on top of 8208 XBR
* 100gr Barnes TTSX on top of CFE223
* 129gr Nosler Accubond Long Range on 30.7gr CFE223
* 90gr TNT Federal factory ammo (50rd boxes, does 2900fps from the 17.6” BTW)
* 120gr OTM Federal factory ammo
* 123gr ELD-M Hornady factory (I’ve been shooting this the most lately.)

Other than some light brass marks on the bolt face, the bolt looks new.

I’ve been shooting the 12” Grendel with CLGS, exclusively suppressed for 4 years now. It has one of the Grendel Monster logo bolts I got in 2017 I think. Looks new. I built it with the Bootleg Adjustable Gas Carrier and have run it on the fully suppressed (choked) position ever since it got the TBAC Ultra 5 Ti suppressor.

I’ve mostly shot 90gr TNT, 120gr Federal OTM, and 123gr ELD-M through it so far.

Since this happened with 2 different bolts for you, I’m thinking the surface hardness of your barrel extension and the TiN is doing something, and might not be machined right. Proof makes high quality components in my experience, so maybe this is a fluke.