Bolt failures

[jmart]

Does anyone mfg barrels with a more erosion-resistant bushing installed at the gas port location? I'm thinking of something like a 17-4 alloy bushing installed in a conventional CM barrel, then having the boring and rifling operations performed.

[Dano5326]

Gas port, throat, and overall barrel erosion would probably preclude such an expensive manufacturing process.

I would like to see a barrel plated after the gas port was drilled, I wonder if the port would hold up longer, or the how the chrome/nickel boron/whatever would hold up.

At some time you easily reach a point of diminished returns vs. price. Weapons are just tools with a lifecycle replacement cost, that must be considered.

[twl]

Our way of thinking has been, if you know it's going to happen, and you can't stop it, but you want to have uniform gas function over the life of the barrel, then you install a gas regulator on the system(external to the barrel), and regulate the gas properly and periodically as gas port erosion progresses.
We've been doing this since the mid-90s, with good results.

When Mack(Gwinn) and Jim(Sullivan) got together back then and worked on the carbine reliability package of Adjustable Gas Tube, RRB buffer, and D-Fender D-Ring, they really did a nice job of identifying the carbine issues and dealing with them. Short of lengthening the gas system again, these things are the logical steps to take for the carbine gas system.

And yes, it will help with the stresses on the bolt, which just might improve its life too. Depends on many things, but it isn't going to hurt it.

[Bill Alexander]

Sad though it is, the analysis of bolt failure mechanisms is somewhat of a pet subject of mine, especially as my bolt designs are somewhat different from the norm.

If I might first reference the excellent paper from V.Y. Yu*, J.G. Kohl, R.A. Crapanzano, M.W. Davies, A.G. Elam, M.K. Veach at the US military academy, this study deals specifically with the loading of the lugs and is related to lug failure as opposed to the other problem which is the breakage at the cam pin hole. I was particularly pleased to see that they emloyed a Von Mises analysis which is a strain energy method. This is well suited to examining the fatigue loads that act upon the bolt. If I might fault the work I would say that while the loading was well demonstrated the constraints were not and one is left wondering if they applied simple ridgid restraints or spread restraints rather than contact surfaces at the lug surfaces. This said the study does illustrate nicely the strain concentration levels that the geometry of the lugs might create. The conclusions illustrate some of the problems in modelling such a complex structure and load dynamic in that the observed strains are not high enough to provide the root cause of the lug failure and while the observations concerning stress corrosion are valid they perhaps a little of a distraction. For those who are looking at this thread without a background in FEA and fatigue the study is still worth examination in particular the illustrations. These provide a graphic illustration of how the lugs on a bolt concentrate the force that the catridge applies. Very roughly the lugs see 5 times the stress that the cartridge imposes on the bolt face.

When considering the failure of the bolt lugs it is typical to consider the port pressure as the sole influence on this event. Quite correctly increasing the port pressure will increases the carrier group accelleration rates but by rational why should a faster carrier speed reduce the bolt life when the headthrust from the cartridge is identical (see the above analysis): the faster unlock cycle also now imposes additional sideways bending loads on the bolt lugs as the residual pressure in the cartridge case will push backwards as the bolt rotates. To illustrate, the Mk18 and M4 bleed gas at identical points but the Mk18 has less barrel length after the port. While not totally correct to make a direct comparison due to changes in the internals that came from the problems with the M4 and the fussiness of a 10.5" barrel, the Mk18 is showing a better bolt life than the M4. So when analysing bolt life one must consider not only the port pressure and the chamber pressure but also a very old term which is bore time. Thus regulating the pressure that the gas tube transmits to the carrier group will decrease accelleration and to some extent elleviate the problem of thrust during unlock but ultimately it will also decrease reliability of the cycle. The solution is therefore to change the timing. Additional bolt group weight is a good start.

I do find the observations regarding the training ammunition very interesting. The chamber pressure if truely around 75,000 psi is duplicating the proof loads. Typically these will fail a system within 30 rounds. I am assuming the light bullets require the extra pressure to generate enough port pressure given the very short duration.

The last area to consider in the failure of the bolt is cracking at the cam pin hole. Beyond the metalurgy of this thin section higher carrier group acceleration will impose increased load at this area but the preperation of the edges particularly where the swage marks are applied must be considered. The fit between the cam pin and the bolt is also a major source of loading for this area. Typicaly cam pins run to the small side and the cam pin hole in the bolt is to the large end to allow easy assembly. This also imposes bending loads on the bolt and a much tighter fit while not as easy to use in the field will extend the life of both the bolt and cam path (assumng the basic physical properties are present, you cannot compensate for crap). The down side for the production is that the parts must be better made to avoid problems with distortion during heat treatment., so cheap parts with open tolerances rule. Changes to the cam path over the length involved can provide little relief from the forces imposed, but as with the lug loading, increasing the carrier group weight while maintaining good port pressure will help to compensate for timing issues without sacrificing reliability.

Bill Alexander

[Submariner]

Bill-What effect, if any, does shot peening bolts have on reduction of stress to the lugs?

[Bill Alexander]

Gas port, throat, and overall barrel erosion would probably preclude such an expensive manufacturing process.

I would like to see a barrel plated after the gas port was drilled, I wonder if the port would hold up longer, or the how the chrome/nickel boron/whatever would hold up.

At some time you easily reach a point of diminished returns vs. price. Weapons are just tools with a lifecycle replacement cost, that must be considered.

Gas port errosion while inherent in the design of any gas operated weapon should be a consideration of the design such that the changes that take place during the weapons intended life do not influence either the life or the reliability of operation. If the port of the M16 type weapon is correctly positioned the errosion that takes place is somewhat offset by fouling that accumulates in the gas tube and the dimensional changes that occur between the gas key and gas tube. This is one area that increasing the carrier group weight does little to correct over the weapons life but good steel in the barrel and attention to the finish on the port will help. Chrome plating after the gas port is present is a viable technique and is neither difficult or expensive.

I would agree with TWL that an adjustable gas system is ideal but as with the M249 the adjustment will be adjusted simply because it exists and as with most things that are issue equipment more is always better. Such misadjustment will undoubtably cause more problems than they solve.

For a general issue weapon it is usually better to simply establish the usable life and then schedule replacements, than to try and compensate to extend the life. Given the general wear on other parts from the abrasives that will contaminate the gun from the environment a life cycle of +12,000 simply incurs weight and cost, that cannot be supported by barrel durability. I am still mystified that military armorers actually work on these things. At a vehicle level we used to condem tracks well before the limits were reached if the vehicle was to operate in a hostile environment,rather than risk failure. The attitude was that at $44,000 a set, chuck it in the trash rather than risk the crew. For a rifle that runs below $2000 but is so mission critical why do we monkey around. Yes it should give an acceptable field life but it should not be an excercise in allowing politicians to make savings.

This still does not address the issue of the overpressure frangible ammunition. A training round should not run a service life less than operational rounds.

Bill Alexander

[Bill Alexander]

Bill-What effect, if any, does shot peening bolts have on reduction of stress to the lugs?

Shot peening induces a compressional load in the suface of the material which will help to resist the formation of surface cracks. The stress that is imposed on the lugs is only slightly reduced at the material surface by the action of this compressional stress. However one must also consider that the bolt also carries a carbon case hardening which also induces compression so the effect of peening is very moot.

I would always advocate that the final finish on the bolt is by grinding but that this is refined by a burnishing before the bolt is bead blasted for the application of manganese phosphate. Phosphate should be carefully applied to avoid differential etching (probably the corrosion pits identified in V.Y. Yu*, J.G. Kohl, et al) and ideally post phosphate baked. Under no circumstances should ammonia based cleaners be employed in the vicinity of the bolt.

Bill Alexander

[Submariner]

Shot peening induces a compressional load in the suface of the material which will help to resist the formation of surface cracks. The stress that is imposed on the lugs is only slightly reduced at the material surface by the action of this compressional stress. However one must also consider that the bolt also carries a carbon case hardening which also induces compression so the effect of peening is very moot.

Bill Alexander

Thanks, Bill. Why, then, is shot peening done to the bolts produced by Colt, FN and LMT?

[Bill Alexander]

Thanks, Bill. Why, then, is shot peening done to the bolts produced by Colt, FN and LMT?

The biggest reason is that it calls this process up on the current inspection drawings. This aside the process is not detrimental and every little helps. The bolt surface must be prepared prior to phosphate and better to peen or beed blast than to abrasive blast. I certainly bead our bolts

Bill Alexander

[Low Drag]

This post is educational, as Marine I just used and maintained (field) an M16. This topic is something normally covered by armors.

Based upon the conversation here I think I can safely conclude the following:

The shorter the barrel the greater the pressure on the BCG. Hence a shorter life cycle, same for barrels due to heat induced erosion near the gas ports.

There is an inverse relationship between barrel length and the two affects above.

There are devices to mitigate pressure on the BCG but nothing will reduce the erosion near the gas port.

Since I've seen evidence from SOPMOD these bolt & barrel failure occurs on issue M4s between 4K and 10K rounds I can expect this from nearly every M4ish type carbine. Of course with a shorter barrel, 14.5" or less the affect happens sooner.

Am I on track here?