When you rebarrel your .308WIN AR10 to 6mm Creedmoor, it’s a good idea to make sure you’re running a high pressure bolt.*
* yes, work was done by well established AR gunsmiths and bolt was properly head spaced.
Bolts: Just a reminder…
- Thread starter Dogtown
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When you rebarrel your .308WIN AR10 to 6mm Creedmoor, it’s a good idea to make sure you’re running a high pressure bolt.*
* yes, work was done by well established AR gunsmiths and bolt was properly head spaced.
Honestly I don't know. It came with an upper a friend gave me about 8 years ago. It ran like a champ for many years consuming .308WIN, but broke after test shot number 3 with 6mm C. These loads were really mild sighters (Berger 95VLD w/ H4350), so I figured the bolt was probably a bit worn out and just didn't like the new cartridge pressure.
My understanding is HP bolts have smaller firing pins to prevent extreme cratering of primers or potential puncture.
EDIT: Here is JP's marketing claims:
Whether we want to admit it or not, the good old Mil-spec bolt has had a good run, but it's long past its retirement age, and the JP EnhancedBolt™ is there to pick up where it left off.
Material matters, and the Carpenter 158 steel of the standard Mil-spec bolt is suboptimal for such a high-load application leading to a life expectancy of only about 6,000 rounds. In fact, standard bolts will start to show stress cracks on the locking lugs adjacent to the extractor cut after only 3,000 rounds while cracking at the cam pin hole and weak extractor spring tension only add to the problems of the Mil-spec bolt. Fine enough for its time, the material used in these standard bolts is simply not the best choice by today's standards.
The SAE 9310 of the JP EnhancedBolt™ is a superior steel alloy in virtually every way compared to the C-158, whether in its resistance to structural failure or life expectancy, both of which far exceeding the Mil-spec. Just as with the transmission gears for Formula 1 racers which are also made from 9310, the operational demands of a bolt assembly are ideally suited to this state-of-the-art material, with actual full-auto endurance torture tests prove it.
Beyond its material advantages, the design of the JP EnhancedBolt™ boasts many other advantages, including the addition of material around the cam pin hole to eliminate this inherent weak point of the bolt design itself. Each bolt also comes assembled with our proprietary Enhanced Ejector, Enhanced Extractor (.308 and Grendel) and Enhanced Gas Rings for improvements in every subcomponent. Yet even with these advanced components, all EnhancedBolts™ (excepting our .308 high pressure model, for obvious reasons) are fully compatible with the readily available Mil-spec subcomponents, including a standard configuration extractor—all the benefits of a state-of-the-art proprietary platform with none of the inconvenience.
My reminder is that every time somebody builds an AR15 upper you'd always see someone saying use a new bolt. Large frames, never. Unless they're buying a complete brand new BCG. Then, it doesn't run so we get all the details, lol.
I grabbed the wrong part:
"Manufactured to the exacting standards of our standard bolt offerings, the High Pressure EnhancedBolt™ design represents a giant leap forward in alleviating the problems traceable to over-pressured loads—namely primer flow, ejector smear, extraction issues, and ignition reliability. In combating these problems, this new bolt design opens up a much broader range of ammunition compatibility and load density in the AR .308/7.62 and other variants such as 6.5mm or 6mm calibers in this platform.
The critical advancement of this bolt comes in the reduced diameter firing pin orifice that precisely accommodates the .062" tip of our proprietary low mass firing pin. This modified relationship virtually eliminates primer flow and the possibility of pierced primers on any SAAMI or NATO specification ammunition while the reduced mass of the firing pin also improves lock time and ignition reliability with hard primers due to the concentration of kinetic energy on a smaller impact area. This is especially advantageous with some aftermarket match-type fire control systems that may use reduced-power hammer springs."
This kind of reminds me of doing a trigger job on a Glock only to have it go bang 99/100 times. Sure, it works and shoots great, but in the back of my mind I would always be wondering if it's going to fail when I need it, so I just put the factory trigger back in and deal with it.
JP bolt ordered.
JP bolt ordered.
Preferably also one that has been magnetic particle inspected for cracks.
Other than potential differences in quality, the issue you saw isn't something addressed by a HP bolt.
I'm thinking your issue had little to nothing to do with headspace, and everything to do with gas system timing. In short, your rifle was most likely overgassed pretty bad. Either that or the bolt was ready to let go anyway (from the previous barrel being overgassed pretty bad), which sounds pretty likely if it gave way on the 3rd shot.
The "higher cartridge pressure" thing is mostly a falsehood. In reality most of us who handload are going to run a 308 at about the same pressure as a 6mm Creedmoor or anything else based on the same brass and primers, because those are what show the pressure signs* that most of us look for. What does change is not pressure in the chamber, but pressure at the gas port and the length of time the port is exposed to that pressure gradient. That may sound like engineer nerd stuff depending on your background, but in essence - the different bore size and powder burn rates may mean your rifle is more or less overgassed (typically more with a smaller bore), and that is not caused by cartridge pressure but by gas system details.
Personally, if that were my rifle I'd install a good quality bolt, and then spend some time really tuning the gas system well before doing anything else with it. That could be with an adjustable gas block, restriction bushings in a fixed gas block, different buffer weights, or a handful of other options; some are better than others but the main thing is to get it tuned right so the carrier isn't yanking that bolt out of lockup while case pressure is still holding it in place.
*Acknowledging here that primers are a terrible way to judge pressure, and some of the common "pressure signs" in brass are also caused by headspace due to excess case sizing.
What he said. "High Pressure" AR308 bolts just have a smaller firing pin aperture, they don't actually hold more pressure. 6CM and 308Win are all the same max pressure.
“Per the manual”? That means literally nothing at all in relation to how your particular rifle is tuned. It may likely still be severely overgassed. The whole point is to tune YOUR rifle.
Start with the gas block fully closed and open it up in small increments until the bolt locks back.
Yeah. I’m aware of their bleed off gas block stupidity. Sigh.
If you want to do yourself a favor, use it in restriction mode as I described above. Or not. Either way, you have to actually tune it for your rifle; leaving it at their recommended setting is most likely why you broke the bolt in the first place. Their implication that all manufacturers size their gas ports a certain way is pure bs, and misleading to those who don’t know any better.
If it helps to understand - a corollary to this would be installing a scope and leaving it set at the factory centered position, without ever zeroing it to your rifle.
If your barrel is severely overgassed, or you’re using a lighter bolt carrier, buffer, and/or spring, you’ll need less gas into the carrier than a different setup might. The fact that you broke the bolt implies this may be the case. Every rifle is a little different.
The other thing to understand is that if your barrel is severely overgassed, that bleed off system may not be able to fully account for it; you’ll have to use the gas block in restricted mode (which IME is better anyway.)
If your barrel is severely overgassed, or you’re using a lighter bolt carrier, buffer, and/or spring, you’ll need less gas into the carrier than a different setup might. The fact that you broke the bolt implies this may be the case. Every rifle is a little different.
The other thing to understand is that if your barrel is severely overgassed, that bleed off system may not be able to fully account for it; you’ll have to use the gas block in restricted mode (which IME is better anyway.)
Noted.
I appreciate the feedback. In all my years in the Army I never dealt with an adjustable gas block and in this particular case the smiths who did the barrel work and upper assembly are highly reputable, so silly me just assumed it was all good to go. Your recommendation makes practical sense.
I appreciate the feedback. In all my years in the Army I never dealt with an adjustable gas block and in this particular case the smiths who did the barrel work and upper assembly are highly reputable, so silly me just assumed it was all good to go. Your recommendation makes practical sense.
The bolt might have worn in to the old .308 barrel after running like a champ for 8 years
The rebarrel and 3 shots and blam?
Crack, crack, break?
Same cam pin?
By 'properly headspaced' you are saying the gunsmith installed the extension on the barrel or did a 'Go-Nogo check?
8 years on a well worn bolt is not saving money in the long run.
The rebarrel and 3 shots and blam?
Crack, crack, break?
Same cam pin?
By 'properly headspaced' you are saying the gunsmith installed the extension on the barrel or did a 'Go-Nogo check?
8 years on a well worn bolt is not saving money in the long run.
Looks like a BCA bcg. Whoever built that should have used the pin gauges to check the bolt. It would have been caught before you got it.
Shoot a AR long enough and parts will break. It is not unusual for this, more so in a big gas gun. Short barrel AR-15 with carbine gas is tough on parts also. Mag dumps are murder on the short guns. Full Auto will eat bolts pretty easily.
This is true. But there are things we can do to prolong bolt life. Tuning the gas system is #1, and doubly so for a large frame.
Lapping the upper receiver and/or installing the barrel extension with a press fit or thermo fit can prolong bolt life too by making bolt lug contact more uniform. Although it wouldn’t do anything for this particular failure.
I appreciate that you recognize my comments as constructive criticism and not just criticism. My intent is not to be hammering you down; my frustration is with Superlative because I hate to see anyone get suckered or misled.
Adjustable gas blocks are becoming much more common the last few years, but there are still a lot of people who don’t understand how they work or how to benefit from them, so you are not alone.
The good news is that if your rifle is currently over gassed; getting it tuned right will make it feel so much smoother and more pleasant to shoot. Once you find a quality replacement bolt of course.
One other comment - some people mentioned headspace, but a lot of guys fail to understand what it really is and why/when it matters. I don’t see any indication that this is a headspace issue, but if you’re bumping shoulders correctly for your chamber, then your headspace is correct with that brass. Even if it were technically out of spec from a go/no-go gauge. Headspace is a function of the difference between your brass and your chamber so as long as you’re sizing correctly, it doesn’t really matter what a gauge tells you. Conversely, if you aren’t controlling shoulder bump well, your headspace may be excessive even if a gauge says it’s within spec.
Hope that helps.
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Yup, don’t think it’s a headspace issue. This is with virgin brass, so I haven’t bumped anything yet. 20 years ago I would have accidentally over/undersized brass, but not so much these days.
At this point I’m chalking it up to a worn out bolt and being over gassed. I’ve dialed the gas regulator all the way down and will just open it up shot by shot until it is no longer flinging brass like a G3.
At this point I’m chalking it up to a worn out bolt and being over gassed. I’ve dialed the gas regulator all the way down and will just open it up shot by shot until it is no longer flinging brass like a G3.
I have several SA AGB's and I've gone back to just restrictive mode on all of them, the Bleed Off mode is very inconsistent. To all that haven't already learned this lesson just buy a basic AGB, they all get carbon locked at one point or another so just pick a quality basic set screw design. I personally like the Seekins offerings.
Side Note.. if you wish to help mitigate carbon locking just get some Permatex Anti-Seize and apply it to the threads adjustment screw.
Yep. Set it and forget it. Then use something like a Bootleg adjustable carrier to switch between open and suppressed if needed.
I have yet to try them, I've always thought that restricting the gas at the source (Gas port) was more efficient especially in reducing carbon deposit in the action and BCG.
I'm in the process of rebarring a OBR to a 18" Rifle length 6.5CM so maybe I give it a go then, but I don't shoot suppressed so kind of a moot point I guess but it would be interesting to see how using both forms of restrictions works.
The gas block (like a Seekins, etc) is much more effective for tuning the rifle unsuppressed.
Once that is done, the Bootleg carrier allows you to switch the gas system between unsuppressed and suppressed settings. There are only 4 settings though, and the range of adjustment is relatively small. They are pretty specific to suppressor use, and are not very good for general gas system tuning if you aren’t using a suppressor; that is better done with a gas block.
I have 3 or 4 of those Bootleg carriers, but only use them in rifles that see both suppressed and unsuppressed use. I wouldn’t buy one for any other application.
Thanks for the guidance, guys. JP HP bolt ran flawlessly, and I tuned the SA bleed-off gas system as recommended. It locked open a little over a full rotation from closed, so the default setting of 4.5 rotations was way over gassed as suggested.
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