why would bolt break?

[dammitman]

OK, after a afternoon of shooting the same brass and primers but starting with 28 grains cfe223, i am convinced that the bolt just broke. I guess it just happens. Final decision is keep extra bolts.

[Tedward]

OK, after a afternoon of shooting the same brass and primers but starting with 28 grains cfe223, i am convinced that the bolt just broke. I guess it just happens. Final decision is keep extra bolts.

Call Les Baer and get one...

[dammitman]

I ordered 2 on monday and recieved them thursday. One stripped and one complete. I started to tell them the story but they told me that because it was not a complete les baer rifle there was no warrenty.

[LR1955]

I ordered 2 on monday and recieved them thursday. One stripped and one complete. I started to tell them the story but they told me that because it was not a complete les baer rifle there was no warrenty.

Handloading also voids the warranty.

[Bill Alexander]

The issue of bolts is complex. Steel type is the beginning and end of a saga but the interaction between a bolt and a barrel extension can negate all the good work elsewhere. I routinely inspect barrel extensions and it is not unusual to see the pitch diameter be off axis to both the centre line of the part but also with marked angularity to the lug surface. Sure, the barrel shoulder should square everything up but if you pull a class 3 thread on a 60 HRC part up to a soft shoulder on the barrel the final result is more luck than judgement. Seen more than a few barrel makers fix the problem with loose threads, lots of problems. Have fun with that build!

The Grendel is exactly what it says on the box. We shoehorned a bigger cartridge into the gun using the tolerances and clearances. As such you have to respect that the parts now have to be right on the money both dimensionally and also mechanically. Something drifts and you will get problems. The epitome of this is the magazines. A correctly dimensioned magazine will function without flaw. Let the spring width drift and the thing becomes an instant doorstop. Wasp waisted mags or barreled mags, same problem. QC has been the nemesis of the magazines not the base design. Something we intend to fix in the very near future with a new magazine.

Bolt steel is as complex a topic as bolt geometry. C158 is a go to selection but lacks some attributes in heat treatment that are specific to the manufacture of Grendel bolts. It still makes a good bolt and has the lowest dimensional movement but temperature control and atmosphere in the oven becomes a very disruptive facet of life. AISI 9310 is not quite the answer. It can have problems with long term life even when subjected to correct loads. This said, there are a lot of very good steels out there and you can trade out performance parameters and heat treatment solutions to get where you need to be. At the far end of this is still AISI 8620. Essentially a surface hardening gear steel the material can make excellent bolts but there is little leeway from a perfect heat treatment. Now impose a Grendel geometry and you are right at the tipping point.

And finally hand loading. LR is correct that the the Grendel must be approached with caution. If you simply load it up and then look for flat primers you have already killed your parts. The MAOP is a very mild 50,000 psi, proof is 67,340 psi. I cannot see the difference on the case or the primers when firing the two. That said it is quite capable of meeting the performances we built in while remaining durable. I have test mules that have run over 8000 rounds and are still ticking along. But a small case will bite without much provocation. I know how my throat works and we set up the loads and OAL to suit. Anyone ever notice that the 100 Berger could be loaded out longer than the factory set up? We set the jump to control the pressure. Same with the 123 Scenar. Sub a 120 SMK and then set to the lands minus a bit and the pressure will skyrocket with very little load adjustment. Tiny case = sensitive.

My final observation relates to CFE 223. I believe this powder needs more examination. Compression seems to have the potential to create pressure spikes but I cannot confirm this. The large primers which seem benign with other powders might be a much larger variable. As with the Grendel itself which leverages more power into a small frame so do these new high performance powders lever more energy into less material.

[woohoo]

Is your receiver square and all lugs bearing on the extension lugs? If the front of the receiver is out of square it can cock the extension in the receiver. Mega and Vltor receivers have a smaller bore and fit the diameter of the extensions tighter which assures the extensions are square with the bore. You can brush a little Dykem on the back of the bolt lugs, shoot the rifle and inspect the back of the lugs to find out.

[SHORT-N-SASSY]

The issue of bolts is complex. Steel type is the beginning and end of a saga but the interaction between a bolt and a barrel extension can negate all the good work elsewhere. I routinely inspect barrel extensions and it is not unusual to see the pitch diameter be off axis to both the centre line of the part but also with marked angularity to the lug surface. Sure, the barrel shoulder should square everything up but if you pull a class 3 thread on a 60 HRC part up to a soft shoulder on the barrel the final result is more luck than judgement. Seen more than a few barrel makers fix the problem with loose threads, lots of problems. Have fun with that build!

The Grendel is exactly what it says on the box. We shoehorned a bigger cartridge into the gun using the tolerances and clearances. As such you have to respect that the parts now have to be right on the money both dimensionally and also mechanically. Something drifts and you will get problems. The epitome of this is the magazines. A correctly dimensioned magazine will function without flaw. Let the spring width drift and the thing becomes an instant doorstop. Wasp waisted mags or barreled mags, same problem. QC has been the nemesis of the magazines not the base design. Something we intend to fix in the very near future with a new magazine.

Bolt steel is as complex a topic as bolt geometry. C158 is a go to selection but lacks some attributes in heat treatment that are specific to the manufacture of Grendel bolts. It still makes a good bolt and has the lowest dimensional movement but temperature control and atmosphere in the oven becomes a very disruptive facet of life. AISI 9310 is not quite the answer. It can have problems with long term life even when subjected to correct loads. This said, there are a lot of very good steels out there and you can trade out performance parameters and heat treatment solutions to get where you need to be. At the far end of this is still AISI 8620. Essentially a surface hardening gear steel the material can make excellent bolts but there is little leeway from a perfect heat treatment. Now impose a Grendel geometry and you are right at the tipping point.

And finally hand loading. LR is correct that the the Grendel must be approached with caution. If you simply load it up and then look for flat primers you have already killed your parts. The MAOP is a very mild 50,000 psi, proof is 67,340 psi. I cannot see the difference on the case or the primers when firing the two. That said it is quite capable of meeting the performances we built in while remaining durable. I have test mules that have run over 8000 rounds and are still ticking along. But a small case will bite without much provocation. I know how my throat works and we set up the loads and OAL to suit. Anyone ever notice that the 100 Berger could be loaded out longer than the factory set up? We set the jump to control the pressure. Same with the 123 Scenar. Sub a 120 SMK and then set to the lands minus a bit and the pressure will skyrocket with very little load adjustment. Tiny case = sensitive.

My final observation relates to CFE 223. I believe this powder needs more examination. Compression seems to have the potential to create pressure spikes but I cannot confirm this. The large primers which seem benign with other powders might be a much larger variable. As with the Grendel itself which leverages more power into a small frame so do these new high performance powders lever more energy into less material.

Bill, thanks for your comprehensive, no-holds-barred Reply: ". . . We shoehorned a bigger cartridge into the gun. . . . Now impose a Grendel geometry and you are right at the tipping point."

[dammitman]

Is your receiver square and all lugs bearing on the extension lugs? If the front of the receiver is out of square it can cock the extension in the receiver. Mega and Vltor receivers have a smaller bore and fit the diameter of the extensions tighter which assures the extensions are square with the bore. You can brush a little Dykem on the back of the bolt lugs, shoot the rifle and inspect the back of the lugs to find out.

I am going to do this. But what if i find it is out of square? What else should i be looking for? I changed my mind, something else is really wrong eith this rifle. Another bolt broke.

[pinzgauer]

You are correct but let me just interject something as well.

Yes, 5.56 bolts will shear lugs but the number of times is miniscule compared to Grendel and 7.62 X 39 bolts in Grendel chambers.

And I have not found any pattern to this type of thing.

I think the cartridge needs to be loaded within a much narrow set of standards than other sporting cartridges or the bolt will give up the ghost much faster than a person should expect.

Agreed on all, LR. And I should have pointed out, typical 5.56 bolt service life is still probably 2-3x the barrel life.

Just pointing out it's not an indefinite life item like a Mauser bolt.

Also, as I understand it, we can't extrapolate from M16 experiences as the carbine gas system does have a higher stress to the bolt.

Does not change bolt thrust issues, but does have a more violent unlock. And apparently they are seeing higher incidences of bolts breaking at the cam pin. And the common overgassing of non-tdp guns increases the risk.

Tedward, yes MPI does little without proof testing other than to confirm no defects in the materials. My understanding that it's real value comes after firing of the proof load.

Bill Alexander, thanks for weighing in on the metals discussion. As usual you add insight that many miss. I still refer to one of your old posts on 4140 vs 4150 barrels periodically.

OP, if you've broken another bolt, and it's with factory ammo, then something is wrong. (Which I think you've already figured out)

If you are seeing high pressure signs on the brass, then I'd suspect leade / "short chamber" issues.

If not, then I'd suspect improper lug engagement/machining causing unequal bolt thrust. (But there are many far more qualified to comment, that's just where I'd start)

Unless you are getting bolts from the same production run I'd find it very unlikely that you had two bad bolts in a row.

[woohoo]

I am going to do this. But what if i find it is out of square? What else should i be looking for? I changed my mind, something else is really wrong eith this rifle. Another bolt broke.

Buy a tool to square the receiver from Brownells or use a Mega or Vltor receiver.

IF you have a carbine gas barrel make sure it is not over-gassed. It should have a .058-.062 port in the barrel. If it's larger get an adjustable gas block or one of BRT(Black River Tactical) gas blocks with the sleeved port. The sleeved port works much better than a screw type adjustable block if you know the size you need. You could tap the port in the gas block between the barrel and the gas tube. Screw a 1/8" long set screw (stainless so you can drill through it) in it and then drill a smaller hole through the set screw.

[dammitman]

Its s 20 inch shilen barrel rifle length gas port. I used a DPMS slick side reciever. I am gonna take this upper apart to see what i can figure out. I used what i thought was good parts and funny thing is i went thru 500+ rounds mostly factory then this. What changed? Its depressing.

[LRRPF52]

Two bolts broken in a row-I'm leaning to the receiver face being off like Woohoo suggests. He had a customer who had the same problem, and broke 3 bolts. He finally had them send him the upper for inspection, and the receiver face was at an angle.

If not that, then the bolt material is heat-treated incorrectly.

[Tedward]

Buy a tool to square the receiver from Brownells or use a Mega or Vltor receiver.

IF you have a carbine gas barrel make sure it is not over-gassed. It should have a .058-.062 port in the barrel. If it's larger get an adjustable gas block or one of BRT(Black River Tactical) gas blocks with the sleeved port. The sleeved port works much better than a screw type adjustable block if you know the size you need. You could tap the port in the gas block between the barrel and the gas tube. Screw a 1/8" long set screw (stainless so you can drill through it) in it and then drill a smaller hole through the set screw.

Would it not be easier to just use an adjustable gas block and dial down the amount of gas passing to the gas key?

[woohoo]

Would it not be easier to just use an adjustable gas block and dial down the amount of gas passing to the gas key?

Easier, yes. As I said before I don't like adj gas blocks with the set screw adjustment. I had a JP gas block erode in less than 40 shots. Not the screw, around the screw. $80 gone in 40 shots. Then I had to go make one that worked like I explained. Clint at BRT is now making them with sleeves which is very similar to the way I have been making them for the last 8-10 years. Paladin or Noveske blocks would be better than the set screw adj type.

He says he has a rifle gas which should be really soft shooting. It should have a gas port around .093 or 3/32"
OP,
500 shots ok then broken bolts...lug just ABOVE the extractor broke?
I would square the receiver, bed the extension with blue loctite and torque the crap out of the barrel nut.

[just_john]

DM,
Just finished a build with a 24" heavy barrel on the same DPMS upper that you used. Put a YHM rifle length hand guard on it and looked real pretty. But, then I screwed in the ring to keep the bugs out of the handguard and found that the barrel was NOT straight in the upper. I could not tell by just looking and would never have known were it not for the ring. So, for me, the obvious answer is that I would have quickly broken the bolt. The barrel is very heavy and mounting into the upper and insuring that it is coaxial is gonna be a real trick. Am trying to create a 'fixture" to make sure that it all lines up. Just torque isn't the answer - the bearing surface is too small. Gonna try standing it vertically when I glue / screw it down.