Recent Army M16/M4 bolt issues

[ExplorinInTheWoods]

Part of the problem is the new m855a1 rounds. It w has a higher chamber pressure than m855 and the only unit using a mid length gas system is SF. The guys at AMU shoot non chrome lined match grade barrels, a good buddy is on the service rifle team. I think it’s kreiger barrels they use. Those guys also have strict records of how many rounds they get. Also units don’t want to do live Ammo turn ins so they’ll do a spend ex and burn up all the ammo. Heating the guns up and putting rounds through them quickly will accelerate the wearing process. The average shooter won’t see themselves changing a barrel or bolt. I know guys on here probably shoot more than average person but a lot of guys won’t see the day their gun is shot out. Also if you’re shooting some of the CHF barrels you’re looking at a 30k round barrel life

[AndyLate]

It's not just the Army. There's a cop on arfcom who claims AR carriers shouldn't be lubed, and he will absolutely lose his shit on anyone who tries to point out the errors in his logic.

The dumber they are, the more certain they are.

Andy

[sinister]

Search for this article and you'll find West Point's engineering department's study of bolt failures:

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


The benefit of cold hammer-forged barrels is economy. Longevity comes with them as a by-product of surface-hardening of the bore, but expecting 30K rounds from an M16 or M4 and still hitting an E-type target at 300 Meters is not realistic.

[Slater]

It used to be generally true that bolts in rifle-length weapons had a longer service life than bolts in carbines. Does that still hold true?

[Ned Christiansen]

The issue with lubed ammo is that it prevents a proper gripping of the chamber walls by the case, increasing bolt thrust. I remember reading that as a kid and it seems so logical that I have always believed it. Increased bolt thrust=reduced bolt life-- I also believe that. I believe these things but I confess I don't know them. Do I go in after cleaning and swab the chamber walls with solvent to make sure there's not a smidge of anything in there? Nope. But I do try to patch it "pretty dang dry".

Seems to me like oil between the cartridge case and chamber wall would actually combust a la Diesel with all that pressure but I've never seen that postulated or discussed.

[lysander]

The issue with lubed ammo is that it prevents a proper gripping of the chamber walls by the case, increasing bolt thrust. I remember reading that as a kid and it seems so logical that I have always believed it. Increased bolt thrust=reduced bolt life-- I also believe that. I believe these things but I confess I don't know them. Do I go in after cleaning and swab the chamber walls with solvent to make sure there's not a smidge of anything in there? Nope. But I do try to patch it "pretty dang dry".

Seems to me like oil between the cartridge case and chamber wall would actually combust a la Diesel with all that pressure but I've never seen that postulated or discussed.

Army testing* of lubricated versus unlubricated ammunition in M4s showed that there is about a 3,000 pound increase in bolt thrust load. This load increase is linear with respect to peak pressure.

Increased bolt loads not only load the bolt lugs, increasing the stress in them, but on the cartridge case head. Bolts are actually fairly stout and the risk of catastrophic failure is slight, but, the case head in much weaker, and therefore deforms in a more alarming manner under increased loads. You may see expanded primer pockets, enlarged rims, and in extreme instances, cartridge case head failure (although this usually needs accompaniment with a secondary problem, such as hot ammunition or some other condition causing over-pressure).

As to dieseling, possible, but highly unlikely - not enough oxygen.
__________________
* Effects of Lubrication and Pressure on Bolt Face Forces, by Michin, South, and Brosseau, US Army Research Laboratory, Oct 2010

[Curlew]

Army testing* of lubricated versus unlubricated ammunition in M4s showed that there is about a 3,000 pound increase in bolt thrust load.

3000 lb increase above what baseline for a dry chamber? I’m guessing something like 5000 or 6000 lb, but I figure you actually know....

[1168]

3000 lb increase above what baseline for a dry chamber? I’m guessing something like 5000 or 6000 lb, but I figure you actually know....

In case lysander doesn’t come back

Thank you, but I will continue to get my engineering advise from actual engineers, with actual degrees.

"Effects of Lubrication and Pressure on Bolt Face Forces", by Michlin, South and Brosseau, Weapons and Materials Research Directorate, Army Research Laboratory, October 2010:

Abstract:
"In this research, the effects that the level of lubrication has on the case mouth pressure and bolt force of a M16 are quantified. The case mouth pressure and resulting bolt force were measured for M855 and M855A1 cartridges as a function of the level of lubrication in a modified M16. The research found that the level of lubrication on the ammunition, and specifically the lubrication between the cartridge and the chamber of the weapon, greatly changes the level of force on the bolt face. The average difference between the conditions of heavy and normal lubrication was found to be 3000 lb. This force is a linear function with the peak pressure of the cartridge case and the peak forces on the bolt occur prior to the unlocking of the bolt and projectile exit. The results of the experiments are presented and the ramifications of the force on the M855 cartridge case are discussed."

For reference, the average normal (measured) force on the bolt at 70 degrees F, with M855 ammunition 2073 pounds, with M855A1, 2560 lbs. At the same temperature, but with a heavily oiled cartridge case, the measured force on the bolt face was 5230 lbs for M855 and 5796 lbs for M855A1

If the ammunition temperature gets to 125 degrees F (which is easy to do if left in direct sunlight for some time) the numbers are:

M855
normal - 2526 lbs
oiled - 5553 lbs

M855A1
normal - 2721 lbs
oiled - 5875 lbs





Outside of the chamber, I will agree that there is no such thing as "too much oil", inside the chamber, things are different.

[lysander]

3000 lb increase above what baseline for a dry chamber? I’m guessing something like 5000 or 6000 lb, but I figure you actually know....

Actual, measured bolt face loads for normal, dry ammunition are around 2,000 to 3,000 pounds.

Well lubricated ammunition shows about 3000 pounds above that, 5,000 to 6,000 pounds.

The normal thumb-nail bolt thrust calculation (case-head radius, squared, times pi times the peak pressure) yields the peak bolt face load with zero friction between the case and chamber wall, (i.e., well lubricated).

Ever wonder how primers pop out?

They cannot be blown out by the chamber pressure, the bolt face would hold them in place until the case is ejected and then the internal pressure of the case is ambient. What happens is: when the case head is subjected to extremely high loads squeezing it between the gas and the bolt face, the head spreads radially. This increases the diameter of not only the rim, but the primer pocket well. If the primer pocket is squished badly enough the primer is no longer an interference fit in the pocket and, quite literally, just falls out of the case during ejection of the spent case.

Case head deformation, and failure, is the real risk of over-pressure and high bolt face loads.

[Curlew]

Thanks for that. So oiling the chamber more than doubles the load on the bolt. Yikes!

With a dry chamber, the brass itself is picking up more than half of the total pressure on the case head, but when you oil the chamber, the bolt has to carry it all.