why would bolt break?

[bigbear_98]

Can someone take a picture of the relief? I guess I don't understand what it is supposed to look like. I just looked at 5 different 556 bolts, LMT, Larue, aero precision, white oak, and rock river, and only see one, the LMT, that has something that looks like a relief.

[bigbear_98]

Aero precision


Rock River


Larue Tactical


WOA

[bigbear_98]

LMT

[SHORT-N-SASSY]

AA made a decision to use the AR15 receiver set for these cartridges. With the Beowulf, it requires a larger ejection port like the other big-bore uppers do. No matter which way you look at it, you have to decide what the market will be able to support.

* With the Grendel shoe-horned into the AR15, you have massive market access because of parts commonality.
* If you decide to build a new receiver set, then why stay constrained to the AR15 mag well COL limitations?

At that point, you are the sole-provider, and you have to convince machine shops to run way more special parts now, unless you think one business has the capacity to make all their bolts, bolt carriers, barrel extensions, magazines, special handguards, etc.

So in order to make it accessible with the most popular rifle design, the fewer unique parts, the better. With the Grendel, that is the bolt, which already has a reputable source for it from the Beowulf production chain, that is properly heat-treated, inspected, and passes industry standards based on engineering work done on it. In short, it already works well.

You can't blame the originator who is doing their part to crank out quality parts that hold up to the pressures, when others who have set out to make their versions work using a different set of standards, results in failed components.

Right now, as long as the metallurgy is done correctly, you have a perfectly functional system that enjoys commonality of parts to the AR15. As soon as we depart from that, we then need new barrel extensions, new barrel nuts, and new upper receivers at a minimum. Why stop there?

The demand for Grendel performance from the AR15 is only going to grow, primarily because of hunting, followed by target-shooting. The best thing for manufacturers to do is to get on the ball on their materials sourcing, heat-treatment, and other critical design specifications.

The bolt in the picture above does not even have the relief cuts behind the lugs that are common to 5.56 bolts, just like many other "AR15" manufacturers have been doing for decades now. Somewhere, someone who specs out the dimensions for their parts is either unaware of the TDP, or chooses to ignore it for whatever reasons, and I'm talking about 5.56 guns here.

With the Grendel and even 7.62x39, there are some engineering challenges that need to be addressed, and AA has done this from the start. Precision Firearms has also been cranking out reliable, durable Grendels, as has JP. I have talked with shops that aren't even mentioned on this forum, like Specialized Dynamics, who caters to a lot of coyote hunters. Number one caliber request: 6.5 Grendel

Precision Firearms has several calibers he builds for: 5.56, 6.5 Grendel, and 6.8 SPC.
Number one caliber request: 6.5 Grendel
Think about how many other shops out there are getting requests for Grendels that we haven't heard of.

I think this deal with the bolt group buy has a lot of potential to establish a regular source of quality bolts, that could blossom into a larger market. Numerous shops and dealers have already tried to buy the entire lot before they are even finished, so that should tell you something about demand.

As I read LRRPF52's Reply, the question that's been haunting me, since I received a discouraging E-mail response from Bold Ideas, this past October, clarifying the reason the planned Colt Competition Rifle "PRO" CRG-20 6.5mm Grendel was discontinued came to mind. What if? What if a specialized company like Bold Ideas, dedicated to excellence, well aware of --- and on top of --- the critical variables discussed in this Thread, had gone ahead with their "PRO" CRG-20 6.5mm Grendel?

[LRRPF52]

If you go to the Colt Competition website, and look at the rifle configurations they are offering, they appear to be targeting the 3-gun action-shooting crowd. There aren't a lot of reasons to use anything but a .223 Wylde chamber in 3-gun since most of your shooting is at close range, with 25-50yd bay stages where you hose-fest from position to position.



Magazine capacity is a huge factor in having shorter timed runs than your peers, and fast shot-recovery is essential to competitive splits. A lot of the 3-gun matches are in the Southeast United States, where you don't have many ranges with longer distances. I used to shoot 3-gun at least once a month in NC and SC, along with other USPSA matches, and the farthest ranges we had were maybe 200yds, but 95% of the shots were within 15yds:



So as a competitor, are you going to chamber in 6.5 Grendel, with less magazine capacity, and more limited ammunition availability compared to .223 Rem? 3-gunners go through thousands of rounds in just a few months if they are really being competitive.

Now out here in the West, we are seeing more and more Grendel's at matches, because we have crazy distances, and a lot of long range facilities. Altitude is high at many venues, and more action/adventure type competitions are becoming popular.

While the multi-gun competition disciplines will see more 6.5 Grendel, I think the biggest and most obvious market is still in the hunting segment. Colt Competition is targeting 3-gunners, and they still are ordering parts for the Grendel, which is odd. If they don't want them, it would be nice if they just made them available to us here on the forum. How fast do you think hundreds of AA Grendel bolts would sell....

[LRRPF52]

AA 6.5 Grendel Bolt, 2.810" long, .135" face recess, going strong since 2009:



Unknown origin Grendel bolt, 2.800" long, .1373" face depth, white spot marking, strange longitudinal tool marks behind the lug roots


Real Colt M16A1 5.56 "MPC" Bolt, w/angle cuts. ~1970's Production


Modern Production, unmarked 5.56 Bolt

[bigbear_98]

What is the purpose of the relief cut? Why would five 556 bolts from reputable manufacturers, 3 claiming "mil spec" not have them?

[Tedward]

I was told the relief cut is the way the mil bit is ran forward on one side of the lug then returns to cut the other side of the next bolt lug. The curve is the mil bit spinning and no purpose. I discussed this with Frank at Maxim and it is just another method of milling. The bits he uses are tapered on both sides so one pass can cut both the left and right lug surfaces. No "Fish Hook" on the bottom of the lug is ok. Material and correct heat treating is the key.

SO I'm told. We'll see when the Maxim bolts are delivered.

[woohoo]

their bolts, bolt carriers, barrel extensions, magazines, special handguards, etc.




The bolt in the picture above does not even have the relief cuts behind the lugs that are common to 5.56 bolts, just like many other "AR15" manufacturers have been doing for decades now. Somewhere, someone who specs out the dimensions for their parts is either unaware of the TDP, or chooses to ignore it for whatever reasons, and I'm talking about 5.56 guns here.


.

The half moon and slashes aren't relief cuts. They are tooling marks/overruns from where the chamfer is cut on the leading edge of the lugs so they will cam behind the lugs of the barrel extension.

[bigbear_98]

So they have no purpose?

I was having a hard time understanding how taking metal away was a way to help strengthen the bolt lug. In my years of welding , any time we made a relief cut, or a curl in the weld, or anything of the sort, was to give the stress breaks a place to stop. I could understand if they were to stop the lug from breaking a larger piece off, but keeping it from breaking I don't get. Being part of the machining process makes more sense.

This brings me back to maybe les Baer has the wrong metal in their bolts, or I got a bad one.

[woohoo]

So they have no purpose?

I was having a hard time understanding how taking metal away was a way to help strengthen the bolt lug. In my years of welding , any time we made a relief cut, or a curl in the weld, or anything of the sort, was to give the stress breaks a place to stop. I could understand if they were to stop the lug from breaking a larger piece off, but keeping it from breaking I don't get. Being part of the machining process makes more sense.

This brings me back to maybe les Baer has the wrong metal in their bolts, or I got a bad one.

...............

[LRRPF52]

The half moon and slashes aren't relief cuts. They are tooling marks/overruns from where the chamfer is cut on the leading edge of the lugs so they will cam behind the lugs of the barrel extension.

Thanks for clarifying. I'm no machinist, so your insight is helpful. At the end of the day, alloy selection and proper heat-treating are the key.

I've been thinking about taking a machining course locally for a while now, since I have a laundry list of projects I would like to create, including my own receiver set design, a different recoil system, and some other projects.

I noticed in bigbear's pics that there appears to be an axial relief groove on the LMT bolt just behind the lug root.

[Tedward]

The half moon and slashes aren't relief cuts. They are tooling marks/overruns from where the chamfer is cut on the leading edge of the lugs so they will cam behind the lugs of the barrel extension.

Thanks for the clarification. I was passing on what I heard so it sounds like I was given correct information but sort of worded it differently, but your response is greatly appreciated and respected.

Keep us in line, you know the AR quite well and your opinion and experience is really great to have on this forum.

[bigbear_98]

Well. Lots of phone calls and lots of long discussions later. I have heard many times from many sources that the lbc bolts have qc issues right now. Nothing we can do will keep them from breaking. Anyone else had issues with the lbc bolts specifically?

[jurassic]

Care to share what sources, and who said they have QC issues? The one in my rifle functions perfectly.

Well. Lots of phone calls and lots of long discussions later. I have heard many times from many sources that the lbc bolts have qc issues right now. Nothing we can do will keep them from breaking. Anyone else had issues with the lbc bolts specifically?