Bolt Failure Points

[Badger89]

I know this general topic has been discussed before, but I have a more specific purpose in mind. I don't want to say too much, but I will say I am working on what I hope is an improved rotating bolt design for M16/M4/AR15 patterned rifles. Although I have never experienced a bolt failure myself, and quite frankly I have not fired enough rounds through an AR15 to expect one, I know they do happen. It seems like the most common failure points people have reported are the locking lugs breaking/sheering off and cracks or complete separation at the cam pin hole. Am I correct in understanding that broken lugs are a result of improper heat treating (hardening went too deep) and cracks at the cam pin hole are simply due to a lack of material at that point? Are there any other "weak spots" on the bolt that are somewhat prone to failure? Has anyone ever had the cam pin itself break rather than the bolt?

I'm also curious to know what kind of a market there would be for an improved bolt. I've heard everything from bolt failure being a rare occurrence to an inevitable event, but it seems like failures are more common with heavily used carbines like the M4 than mid-length or rifle length civilian AR15s. Would you replace your bolts with an improved design if one was available from a reputable manufacturer like BCM, or would you stick with the tried-and-true original design? Does anyone have a strong personal opinion one way or the other?

Lastly, does anyone have a spec sheet for a milspec M16 bolt and carrier? I'd rather go off the actual specs when I get to the design phase rather than taking measurements off a bolt that may or may not be to exact spec.

[Split66]

https://www.m4carbine.net/showthread.php?t=1130

[Zane1844]

Today at the range this guy was kinda yelling at how his 300 dollar BCG just broke. I did not see what exactly happened, but he was pissed. I am sure someone would like the idea of a stronger bolt, but my BCM BCG which was half that price has kept going, but only has 3,000 maybe a couple hundred more rounds through it. . (yeah I try to count.)

[Eric D.]

You pretty much nailed it as far as where the failure points are but I wouldn't say failure is a result of improper heat treatment or other process as long as the failure isn't premature. AR bolts fail pretty predictably around 15k rounds. The failure is primarily due to fatigue. I ran some number for bolt stress awhile ago and if I remember correctly, the shear stress in the lugs came out to about 3,000 psi which is well below maximum shear strength for C158. However, my numbers were based on peak chamber pressure under static conditions while the bolt is locked. In addition to shear stress on the longitudinal axis, the bolt lugs are also subjected to shear stress on the lateral axis and bending stress during locking and unlocking. Although the bolt lugs do have a small radius where they meet the bolt body, that is still an area of stress concentration, as is the cam pin hole.

As far as market for an improved bolt your two big players are LMT with their enhanced bolt design and KAC's E3 bolt, neither of which I have seen a reported failure for. The LMT bolt is expensive and KAC's bolt requires their proprietary barrel extension. I think there would be a large market for an improved bolt if it were a drop-in solution that could be made available for a price comparable to a USGI bolt.

[MistWolf]

An important process to AR bolt life is shot peening to relieve stress.

Armalite shortens the height of the bolt lug opposite the extractor claiming it evens out the stress load and reduces cracking

[Markasaurus]

https://www.m4carbine.net/showthread.php?t=1130

now THAT looks like the ultimate AR bolt, indeed! At first glance the third hole in the bolt carrier would seem unnecessary...but simply imagining the massive pressure the whole BCG is subjected to upon firing, another bleed hole cannot be a bad idea at all. One has to ask oneself why Eugene Stoner in his infinite wisdom did not think of such design tricks to begin with, but remembering two things about the entire AR platform could easily explain it all! 1 - Stoner was first and foremost NOT a small arms designer! His engineering background was in lightweight alloys and composites for AEROSPACE not guns after all, the difference between them could be thought of as that between chalk and cheese...and secondly, the AR 15 was a very hurried redesign of his superb AR 10, a much larger weapon with bigger receiver, barrel, chamber and BCG (notice all the photos of the earliest AR 10s, intended for competition in the US Army contest to replace the M1 Garand, had CHROME PLATED bolts and bolt carriers - indicating Stoner realized fouling was going to he a big problem). In other words, sub-par material strength for the carrier and especially the bolt, was guaranteed to be built in when the large BCG of the AR 10 was simply made smaller for the 5.56 round!

Being a complete AR noob that i am, i have never had a bolt fail. I suspect that many bolt failures occur in soldier weapons that get fired full auto quite a bit, the devastating combination of heat, dirt, and pressure surely plays hell on the bolt of any M16 or AR. That's a no brainer...




on aCould the AR USE a stronger redesigned bolt to fix it's flaws, certainly! Are most makers of AR rifles going to adapt the more expensive but stronger bolts, and are users going to retrofit? The short answer is almost certainly NO. THE current bolt works well enough that most people will see no need if it costs them anything..i have already solved this problem, answer, carry a spare bolt? It fits right in the hand guard after all...and i understand a lot of soldiers in afghan do just that...some even carry a spare BCG, and why not?


I think if you want to make the ultimate bolt, you are not dealing really with the metallurgy of steel. The 15,000 round bolt life quoted above seems as much ass it's possible to get from steel. And you are limited by the shape the bolt must be just to fit and function. For a really improved bolt, i think you are really going to be dealing with titanium, titanium alloy or some other exotic metals since the design limits of carbon steel in an AR bolt have probably been reached long ago.

[sinlessorrow]

now THAT looks like the ultimate AR bolt, indeed! At first glance the third hole in the bolt carrier would seem unnecessary...but simply imagining the massive pressure the whole BCG is subjected to upon firing, another bleed hole cannot be a bad idea at all. One has to ask oneself why Eugene Stoner in his infinite wisdom did not think of such design tricks to begin with, but remembering two things about the entire AR platform could easily explain it all! 1 - Stoner was first and foremost NOT a small arms designer! His engineering background was in lightweight alloys and composites for AEROSPACE not guns after all, the difference between them could be thought of as that between chalk and cheese...and secondly, the AR 15 was a very hurried redesign of his superb AR 10, a much larger weapon with bigger receiver, barrel, chamber and BCG (notice all the photos of the earliest AR 10s, intended for competition in the US Army contest to replace the M1 Garand, had CHROME PLATED bolts and bolt carriers - indicating Stoner realized fouling was going to he a big problem). In other words, sub-par material strength for the carrier and especially the bolt, was guaranteed to be built in when the large BCG of the AR 10 was simply made smaller for the 5.56 round!

Being a complete AR noob that i am, i have never had a bolt fail. I suspect that many bolt failures occur in soldier weapons that get fired full auto quite a bit, the devastating combination of heat, dirt, and pressure surely plays hell on the bolt of any M16 or AR. That's a no brainer...




on aCould the AR USE a stronger redesigned bolt to fix it's flaws, certainly! Are most makers of AR rifles going to adapt the more expensive but stronger bolts, and are users going to retrofit? The short answer is almost certainly NO. THE current bolt works well enough that most people will see no need if it costs them anything..i have already solved this problem, answer, carry a spare bolt? It fits right in the hand guard after all...and i understand a lot of soldiers in afghan do just that...some even carry a spare BCG, and why not?


I think if you want to make the ultimate bolt, you are not dealing really with the metallurgy of steel. The 15,000 round bolt life quoted above seems as much ass it's possible to get from steel. And you are limited by the shape the bolt must be just to fit and function. For a really improved bolt, i think you are really going to be dealing with titanium, titanium alloy or some other exotic metals since the design limits of carbon steel in an AR bolt have probably been reached long ago.

The issue is price to performance. The LMT bolt is, well was before the craze 2-3x more expensive than a standard Bolt, But it did not offer 2-3x the life or improvement. The price increase has to be seen in performance upgrades, most wont spend 3 times as much for a product that is 1.5 times better.

[sinister]

Beefing up the body of the bolt (like the HK 416's) might strengthen the cam pin hole area (perhaps lengthening bolt life where the bolt often fractures and separates).

Not sure what can be done with the locking lugs. While both KAC and LMT cut away some of the lug area, not being an engineer I don't know how removing material enhances strength and longevity.



I've never heard of a bolt carrier failing.

[sinlessorrow]

Beefing up the body of the bolt (like the HK 416's) might strengthen the cam pin hole area (perhaps lengthening bolt life where the bolt often fractures and separates).

Not sure what can be done with the locking lugs. While both KAC and LMT cut away some of the lug area, not being an engineer I don't know how removing material enhances strength and longevity.



I've never heard of a bolt carrier failing.

The LMT bolt I have no idea. From what I undertand the KAC bolt is stronger because it increases the size of the lugs in a few places as well as removing all sharp edges.

[markm]

From:

Failure Analysis of the M16 Rifle Bolt
V.Y. Yu*, J.G. Kohl, R.A. Crapanzano, M.W. Davies, A.G. Elam, M.K. Veach
Department of Civil and Mechanical Engineering
United States Military Academy
West Point, NY 10996, USA

4. Conclusions
The fracture of the M16 bolt resulted from a cumulative effect of high stress concentrations at the fillet radius and the additional stress concentration imposed by the presence of localized pitting at the surface. The bolt possesses many fillet regions which impose numerous areas of high stress concentration. In particular, two fillets experienced higher stress immediately adjacent to the round extractor due to the non-contiguous feature of the bolt. These two specific areas of high stress concentration also corresponded to the same location where failure of the bolt occurred in all fractured bolt specimens. Micrographs obtained from the scanning electron microscope of the fractured surface showed localized pitting at the failure initiation site. In addition, transgranular crack propagation near the pit formations in the fillet regions was observed. The localized pits formed near the locking lugs also served as high stress concentration points. The presence of pits in the material amplified the stresses of the bolt in the locking lug region which already had a high stress concentration due to the irregular geometry of the bolt. This cumulative stress concentration provides a good indicator why the crack initiated and propagated from this region.

The wear observed in the controlled experiment indicates the mechanism of why the corrosion pits formed near the locking lug fillet by exposing the Carpenter Steel 158 base metal to the environment. Vickers microhardness readings near the fillet region show that the bolt was not uniformly case hardened. Comparison of the microhardness readings near the fillet region and 10 mm from this region show a disparity of approximately 100 units. The softer, less carburized region near the fillet contributes to the formation of a wear area after firing just 1800 rounds.