6.5 Grendel Bolt Lug Strength - Structural Mechanics Insights

[PA_Allen]

It seems like the topic of Grendel bolt strength, or lack thereof, comes up quite often in discussion threads. Discussions often center around bolt life, risk of lug breakage vs. load pressure, etc. I thought it might be useful to illustrate some basic solid mechanics calculations to illustrate expected bolt lug yield strength safety factor as a function of internal case pressure (and resulting bolt thrust).

Bottom line up front: With the assumption that all 7 bolt lugs are carrying the load evenly, the Grendel bolt design has a shear yield safety factor above 3.0 (Very healthy margin). If you assume only 1 lug is making contact due to poor dimensional tolerances, misalignment, etc., the shear load will exceed the yield and ultimate shear strengths and single lug failure would be expected.






As can be seen from the data above, the Grendel as designed has a healthy safety margin against shear failure of the lugs. See the highlighted row corresponding to 52 Ksi chamber pressure with SF = 3.2. The calculations assume NO friction between the case and chamber wall, which would reduce bolt thrust and increase safety factor, so these calculations can be considered conservative for UNIFORM bolt contact. Increasing chamber pressure to 60 Ksi reduces SF to 2.8 but would NOT result in bolt lug failure for a single overloaded round (as expected for a robust design!).

In reality all bolt lugs will not contact uniformly and some will be loaded higher than others. That is one reason why the lugs near the extractor are often the weak link in the design. Cumulative fatigue damage at the lug roots over repeated loading could lead to fatigue failure of the lugs, but this should NOT happen for correctly dimensioned parts that are assembled in alignment to ensure approximate uniform contact. From looking at these calculations I would have to conclude that Grendel bolt lug failure can only results from:
1) uneven lug loading that overloads a single or pair of lugs. This could break a lug in short order over a small number of cycles.
2) Poor bolt metallurgy that leads to low strength OR brittle failure from loss of ductility (usually a heat treat problem)
3) Existing defect in the material that leads to fatigue crack growth and resulting failure.

Notice that SF is not a strong function of chamber pressure for reasonable loads. So no, you will not shear off a lug from running a single hot load unless there are some other underlying issues like 1-3 above. This is not to say that you should make a point of repeatly running "hot" loads. Higher chamber pressure does increase stress and will lead to an increased rate of fatigue failure.

As an interesting side note the "Flex" column is a rough estimate of elastic deformation of the bolt lugs during firing (about 0.002"). This is not permanent deformation like you see in the fired brass case. Instead this is an elastic displacement that "springs back" in place to zero deformation after firing.

Another interesting thought experiment for another time would be to consider the increased pressure dwell time for suppressed firearms. When running a suppressor the pressure drop in the chamber is delayed from the resistance from the suppressor baffles, so the bolt can be rotating to unlock while still supporting significant back thrust. The friction between the lugs and barrel extension would exhibit torsional stress in addition to the direct shear stress assumed below which would increase the overall lug stress and decrease safety factor.

I hope this post is found useful and can lend some context for future discussions of bolt failures. Feel free to double check my calculations as they were performed quickly with no QC.
Best,
PA

[BluntForceTrauma]

Excellent info, and thanks very much for taking the time to do this — and for applying your expertise for the benefit of the Grendel Horde!

(Speaking as Admin, this earns a Stick cuz the question often comes up and you've addressed it superbly.)

[Peregrinant]

Yes, thank you from an 'unwashed' barbarian

[Growler]

Excellent post, definitely stick-worthy!

What material did you use for the bolt properties? For Carpenter 158, I’m seeing UTS of 160ksi with a yield strength of 135ksi.

Fatigue cycle life decreases rapidly as stress approaches ultimate tensile strength, so that factor of safety of 3 -while good- is not something to interpret as a green light to experiment with more pressure. You might actually need all of that factor of safety to reach design fatigue life.

On the subject of bolt life, I rarely see any discussion of the barrel extension. We go to lengths to lap receivers for bolt alignment and life, but a sloppy/ out of spec barrel extension could introduce its own misalignment. A bolt failure with low round count might call into question the tomato stake it was used with.

[PA_Allen]

Growler,
Great points. I used properties for AISI 9310 VAR (material used for the Monster bolts) that I found on the web. The yield and ultimate strengths can vary due to heat treat differences, but the values I used are representative I think. Also keep in mind that the properties listed by a supplier are guaranteed minimum capabilities, and the actual nominal strengths will generally be higher that what is listed on the spec.

For sure I agree that the barrel extension could be the culprit for poor bolt lug contact along with other misalignment issues such as a non-true receiver face etc.

As far as fatigue failure goes, a good estimate of stress-based fatigue endurance limit is 1/2 of the ultimate strength, so the SF of 3 or so should provide high fatigue resistance. The types of bolt failures that are generally described as occurring in 10s or 100s of cycles are actually low cycle or very-low cycle fatigue failures from repeated high strain cycles. I think this could only occur for cases that I mentioned in the original post and should not be a concern for a nominal build.
- PA

[LRRPF52]

The torsional load variation as a function of timing and unlocking is an excellent subject to bring up relevant to bolt lug life.

It’s why the gas system really needs to be managed with optimum port diameter and flow rate into the Stoner Expansion System formed by the bolt and carrier.

I like that you mention this with suppressors being a major variable that throws the gas system behavior out of balance.

This is why I use Bootleg carriers for anything suppressed.

I also noticed that cyclic rate on a Ballistic Advantage 12.5” barrel with the smaller port worked really well suppressed, whereas many report unreliable function with the same barrel unsuppressed.

I also like that Growler brings up the often overlooked part, the barrel extension.

AR15 barrel extension tooth length really limits the working pressure of the cartridge once the diameter goes up on the case, unless you’re willing to invest in exotic alloys like AerMet for both the extension and bolt.

I actually like the lower pressure behavior of 6.5 Grendel where things aren’t near the edge of getting away from you.

You really see the benefits of this managing the sight picture through the shot, and on brass life if you’re a reloader.

Even if you’re not a reloader, spent cases retain value almost as if they are new because it doesn’t really matter with 1x firing, especially on Lapua brass.