When do Grendel AR bolts break?

[centerfire]

Based on the Henderson Defense thread at AR15.com - a high volume gun rental range that runs them to failure at high round counts; there's a ton of insightful data. One snippet he posted was that carbine bolts break all the time, and they get piles 20 deep every year from that failure. Either lugs, or the bodies themselves shear. Yet he said rifle gas system bolts almost never break. What that implies is that the breakages likely have more to do with the gas system than the actually peak pressure.


https://www.ar15.com/forums/AR-15/High-r...5/?page=17

I would imagine the way the face of a Grendel bolt is cut, lateral forces on the lugs are an extra sensitive item with a Grendel bolt - moreso that direct-face forces are. Which is to say if you whack the end of a lug from the side, a Grendel bolt lug is considerably weaker than a 5.56 bolt lug. But a direct force back on the face of the bolt lug, is likely still weaker, but I suspect relatively less profoundly so.


There are basically 3 stress events on a bolt.
1) Closing. When a bolt closes it has to overcome the ejector spring. Which is to say, the cam-lug wants to start rotating the bolt upon closing, but can't do so until the round bottoms out on the face to fit in the headspace clearance. And it has to compress the ejector before it bottoms out. So while that is happening, the lugs are being rotated against the barrel-extension lugs on the sides, while that spring is being compressed. If the ammo has tight headspace, then the brass will need to be compressed as well, before the bolt can finish closing. It's for this reason I make sure to lube my bolt lugs, so they can slide more easily during this part of the cycling. While I wouldn't call this a particularly hard-wear event - you can see evidence of this on the front corner of your bolt lugs, where there's a little side-wear.

2) Firing. Obviously the big one when it comes to total force. Though all of that force is in-line with the engineered design. Other than catastrophically bad loads, I suspect this is actually a rare point of bolt failure. Just conjecture though there.

3) Cycling. Two items here when overgased, both of them more profound in an overgased carbine gas system.
a) There will just be a lot of force, which will aggressively result in higher BCG velocity, which means violently rotating the bolt. Sometimes bolts snap in half at that weak spot where the cam-pin goes through it.
b) It has been said there can still be some pressure in the gun when the cycling just starts (particularly in a carbine gas system). Not Kaboom level, as the lugs will not clear before all the pressure is gone (ish), but what it means is while the lugs are mashing up against each other, a lateral force trying to rotate the bolt is happening. That's going to encourage bolt lug shear.

I always put a grease like lubriplate (or better)on my lug contact faces for this reason, and always lube the contact surfaces the bolt has with the BCG and cam pin as well. In close to 75,000 rounds in various AR's, I've never broke a bolt lug or broken one in half.

Note also that bolt-lug shear failure is almost always one of the two lugs around the extractor. That's because they're the least supported. Grendel exasperates that, due to the inherently lower supported lugs. It's for this reason some manufactures consider a Grendel bolt a 3000 round life item. It's also for this reason Grendel will likely never be a military adopted round - at least not in the classic AR platform. And since the whole point of Grendel design was to fit in AR platform, it'll likely never be more than it is now - which is a fantastically optimized round that fits in an AR platform, for civilians.

To extend bolt life, I absolutely think running middy or rifle gas-system when you can, lubing your bolt well, and tuning your gas system to not be over-gassed are all good ideas. I will say, I've already broken an extractor at the 600 round mark in Grendel, when out of 75,000 rounds of 5.56, I've also only broken one. That Grendel breakage was in a carbine gassed unit.

Just my own thoughts - I have less experience with Grendel in particular than many of the other posters here.

This seems like the most complete and straight forward explanation. Mechanical failures are a fact of life whether they are user induced or the natural constraints of the system. Plan accordingly and drive on.