why would bolt break?

[bwaites]

Paul,
Why are these Grendel bolts still breaking. You think it has anything to do with hoop strength or is it just how thin the material is around the recess of the bolt? I wonder why Stoner designed the 5.56 bolt to be the size it is and why everyone else is putting bigger cartridges in the same size bolt. You would think they would have increased the size of the bolt in the same proportion that the bolt face was increased. If they had the bolts wouldn't be breaking. I have to assume with the actual breakages reported over the years the rim of the Grendel may be a little to big for a 5.56 bolt.

It would appear that the vast majority of bolts breaking are from new manufacturers. We don't see many AA bolts or Precision firearms bolts break. I suspect that there is a learning curve to building high quality bolts for the Grendels and that it takes a bit to figure out. I know that AA bolts break from time to time, but so do 5.56 bolts, so.....

[JASmith]

Bwaites is close, but there is more of a common thread in the reports of bolt breakage.

Best summarized with this question: Is it new manufacturers or just naive (maybe stupid) use of non-standard loads, chambers, and so on?

There is a reason the SAAMI maximum average operating pressure is 51,800 PSI. That pressure gives about the same thrust as the .223 Remington running at its SAAMI pressure. Hence, broken bolts are almost always the result of human error and not the design of the rifle.

[woohoo]

Bwaites is close, but there is more of a common thread in the reports of bolt breakage.

Best summarized with this question: Is it new manufacturers or just naive (maybe stupid) use of non-standard loads, chambers, and so on?

There is a reason the SAAMI maximum average operating pressure is 51,800 PSI. That pressure gives about the same thrust as the .223 Remington running at its SAAMI pressure. Hence, broken bolts are almost always the result of human error and not the design of the rifle.

Ah but the AR15 bolt was designed by Stoner for a rim diameter of .373". Has anyone done any engineering test to prove they will stand up to a cartridge with a rim dia of .442? Since we actually have plenty of history of the bolts breaking behind cartridges with this size rim I would say no. What do you think Paul, maybe you could run some test on your equipt? Or we can debate this around the net, what do you think?

[NugginFutz]

Wow - a high school style call out! I almost expect a drag race challenge, next.

[JASmith]

Ah but the AR15 bolt was designed by Stoner for a rim diameter of .373". Has anyone done any engineering test to prove they will stand up to a cartridge with a rim dia of .442? Since we actually have plenty of history of the bolts breaking behind cartridges with this size rim I would say no. What do you think Paul, maybe you could run some test on your equipt? Or we can debate this around the net, what do you think?

Wow - a high school style call out! I almost expect a drag race challenge, next.

I think NF be right.

The poster is either terribly naive or deliberately deceptive with his logic.

Bolt thrust calculations are straightforward engineering calculations and have been done many times. They are not, however, done using the rim diameter. The easiest approximation for getting the area is the base diameter but one really wants the largest inside diameter of the case to get things correct.

Any responsible and knowledgeable gun designer will do bolt thrust calculations as part of the cartridge / rifle development. That is the primary reason the Grendel SAAAMI pressure is what it is rather than a higher pressure like the 6.8 SPC or .233 Remington.

The classic proof test is an engineering test and has been done many times.

[woohoo]

Paul keeps referencing all the engineering that has been done on the bolts and how everything is right at the limit for the AR15 rifle. That nothing should be pushed any harder than the Grendel because it is as big and powerful as a case should be because the carrier, extension or upper receiver could fail if pushed further. We never see carriers, extensions or receivers fail only some bolts so the bolt is obviously the weakest link. Years ago we ran some thrust tests for shear strength but that does not tells us about impact and fatigue strength. I don't think the simple shear strength is the determining factor, fatigue and stress cracks are what kills the bolt. I think it would be very expensive to build a jig that would pound on a bolt and extension 50,000 times and produce the same amount of thrust as each cartridge size. Maybe the military has such a tester and Paul has seen it?

[bwaites]

I think that the proof here is in the pudding. There have been very few bolt failures over the last 6 years, except in those who are newly manufacturing the bolts or are using inferior materials.

I also think WooHoo is simply inciting to riot on an issue that is a dead horse. We had this discussion at great length years ago. We will not go back to that point and repeat everything all over again. It did nothing for anyone at that time, and has done nothing for anyone here. The military does not do fatigue testing itself that I am aware of, (why should they, they don't manufacture the bolts!) They would determine the specs, and expect any enterprise supplying parts to follow those specifications. Currently that specification requires proof testing each bolt and MPI testing (Magnetic Particle Testing) of each bolt. Unfortunately, Proof Testing probably actually weakens the bolt, but if it passes the MPI testing afterward, it is accepted.

Both testing forms require extensive and expensive equipment. Most manufacturers do not do that testing on every part unless they are selling parts to .gov/.mil. They test a number of bolts, and if they pass, they may do tests on each run or every 10,000 bolts, or whatever. I know AA has done extensive Proof Testing on bolts at each step of the development. I would hope other manufacturers would do the same, but from the rapidity that some of the recent bolts were produced, I have doubts.

[woohoo]

Does anyone have the calculated figures for what trust a bolt will handle and what the cartridge creates? Of course a 8620 bolt will not handle as much as a 9310 or Carp 158. Hopefully the old 8620 7.62x39 bolts aren't being produced now.

[bwaites]

Does anyone have the calculated figures for what trust a bolt will handle and what the cartridge creates? Of course a 8620 bolt will not handle as much as a 9310 or Carp 158. Hopefully the old 8620 7.62x39 bolts aren't being produced now.

Once again, they aren't useful. If you personally need that information, contacting Colt might be helpful!