Bolt Shear

[K.L. Davis]

Published info is not MTBF stats... but says that bolt failure occures from 3K to 6K in "heavy fire" and 6K to 10K under normal fire.

Sounds like where you are breaking them is normal... you have to judge on the amount of use.

[LukeMacGillie]

Thanks KL,

Looks like im going to be replacing a bolt a week, and a barrel a month from now on LOL

Yes these guys are shooting that much!

[Bill Alexander]

What you are seeing is typical for the M4 set up and is the result of how the rifle is timed with respect to the gas port position and the barrel length that lies in front of the gas port. The increased gas port pressure increases the carrier acceleration rate, which is a good thing with respect to reliability of operation in adverse conditions, but because the residual pressure in the case is enough to produce a head thrust, the lugs are subject to both axial and now bending forces as the bolt rotates during unlock. The result is a multiaxial strain condition rather than a simple shear load and the fatigue life of the two lugs either side of the extractor slot is much reduced. Across a sample of bolts the typical life appears to be between 5000 and 6500 cycles.

This is a general examination and does not account for some of the samantics that are present in the analysis. Corrosion from the use of certain cleaning agents or the enviroment will act to accelerate the condition and this is one of the avenues of examination that the Army has followed. I contend that the initiation of a fatigue crack can be created in a compresion condition as the stress is relaxed and if correct the stress concentration from the geomtry of the extractor cut intersecting with the case recess becomes a critical factor.

Failure at the cam pin hole is initiated by either, oversize hole diameter, undersize pins, incorrect edge condition at the hole or a bad swage at the bottom of the bolt, although this last condition is rare. Diagnosis is easy; look for bight spots top and bottom on the hole through the bolt. The cam pin should not be loose in the bolt after the weapon has been bedded in.

To stop rambling, your bolts appear to be well made items and your failures are what you should expect. Avoid the use of ammonia based cleaners, lube the lugs with a good moly disulphide grease if conditions allow and clean and replace the lube regularly. (use dry teflon in cold/dust). Best insurance is to also use the H2 buffer assemblies.

With this in mind bolts will still fail, establish your failure rate from the fleet as you replace them, and keep track of round counts. Have plenty of spares and if in doubt ditch the bolt and replace it. Bolts are cheap.

Bill Alexander

[Sean King]

I am very ignorant about the AR platform in general......but I"m very familiar with having spares (from my other hobby). That said, do you need a "go/no-go" head space gauge when you change out a bolt?

Also, I have one DI gun and 2 piston uppers now, does anyone forsee more/less/the same amount of bolt longevity from one system over another? If it matters the BM DI gun has a 16" bbl, with a carbine length gas system and has the factory installed buffer (I have no idea what it comes with).

The piston uppers are 10.3" (6.8 SPC) and 10.5" (5.56) and are going on a complete lower from LMT with whatever buffer they supply with their collapseable stock. The 6.8 has an ACB bolt and Raptor extractor, the 5.56 is a standard bolt (pretty sure it's MP tested).

Thanks,
Sean

[Bill Alexander]

I am very ignorant about the AR platform in general......but I"m very familiar with having spares (from my other hobby). That said, do you need a "go/no-go" head space gauge when you change out a bolt?

Also, I have one DI gun and 2 piston uppers now, does anyone forsee more/less/the same amount of bolt longevity from one system over another? If it matters the BM DI gun has a 16" bbl, with a carbine length gas system and has the factory installed buffer (I have no idea what it comes with).

The piston uppers are 10.3" (6.8 SPC) and 10.5" (5.56) and are going on a complete lower from LMT with whatever buffer they supply with their collapseable stock. The 6.8 has an ACB bolt and Raptor extractor, the 5.56 is a standard bolt (pretty sure it's MP tested).

Thanks,
Sean

An obvious consideration but one that I omitted to write about. Yes you should always use a go/nogo/condemnation guage if you change bolts. It is at this stage you begin to see the real differences between top tier bolts and the others. The wear on a barrel extension from a top tier bolt will be even on all lugs so an equivelent new bolt will bed in quickly with very little (0.001" typical) change in headspace over the first 200 rounds. If the gun was previously bed in with an uneven bolt it is very unlikely that the replacement will match the lug paterns worn into the extension so beding in with the new unit might take the headspace to the condemnation limit.

I am afraid I cannot be of much help with respect to your particular guns. With such a selection of unknows your question is sort of like "I have a blue car; what are the gearbox ratios"

DI vs piston influences on bolt life also fall into the same catagory. A badly set up piston will destroy bolts just as well as a badly set up gas tube system.

Bill Alexander

[Sean King]

Thank you for the response Bill. Sorry for the vagueness of my questions. Like I said, I'm very unfamiliar with this platform, so I'm trying to learn as I go.

I appreciate your time and response. It's good to know about the head space gauge.

Sean

[Aubrey]

Bill,

Do you have any insight into how much shot peening or other material/processing options might increase the MTBF of carbine bolts?

[Bill Alexander]

From general good practice the use of shot peening on a high stress low cycle fatigue part will be advantageous to the overall life of the part. How much so is definitely an unknown, as I have yet to find a reference to such a study that also includes data for the shot energy and blasting intensity. Without this all we can say is that on a good part peening is better than not peening.

In terms of material and processing this has a huge effect on how well a part will perform. I have long debated the difference in life of identical bolts but with either hard turned or ground lugs. Given the hard surface and what is observed in cam shaft work it is possible that grinding will actually pre crack the lugs. It is not possible to peen this area due to tolerance changes but it is debatable if the phosphate coating does more or less damage. Despite this I have run both methods of final machining and have not been able to observe a difference in life, but my test sample is way to small.

My rule of thumb is to start with steels that are especially clean in composition and by remit exhibit excellent grain structure. Heat treatment needs to be much better controlled that the .mil specification for this part and the current inspection requirement of surface hardness, case thickness and pressure test with magnaflux should be extended to include residual carbon content in the case, examination of the carbide structure and core structure and hardness. If this is attended to the phosphate coating will not have a tendency to pit the part and the best life should be obtainable.

Bill Alexander

[Hardgear, LLC]

Bill,

Why is magniflux the prefered NDT method?


Bomber

[Bill Alexander]

It is cheap, easy, does not contaminate the part like a dyepen system, and suits the complex geometry of the bolt well. Equally it is easy to train people to get it right which is an advantage if you are on a war footing and need lots of guns now but do not have a lot of labor resource.

If you study the M16 design you can see lots of details that are wrapped up with the WWII concept of every able bodied male is fighting so how do we crank out billions of these things with semi skilled drafted labor using the sort a machinery you see in a car factory. The approach is always solid which is why I am not always too impressed with composite designs. Unless the part size is such that they can be made with common sized injection machines we are going to be screwed if we need lots of them, they can only be made off 8 machines in whole US and two of those are down cause we need parts from Japan/Italy etc.

Bill Alexander