If we're talking boundary layer film thickness I'm a little skeptical.
Now, I'm not tracking here. It sounds like a rehash of the same statement - "the effect that lubing cases has on bolt thrust is minuscule compared to the increased thrust due to lack of friction between the case and chamber wall" is how I'm reading it. One causes the other.
Last edited by Eric D.; 04-14-14 at 17:03.
B.A.S. Mechanical Engineering Technology
Patrick has it right. However, case adhesion to the chamber wall is a highly variable factor and no competent gun designer would use that to determine bolt lug strength. Instead, they simply make the lugs much stronger than necessary. They know that lube will get on ammo and in the chamber. For instance, how many of you use a strong degreaser such as lighter fluid to give the chamber a final cleaning? The first round is almost always from a lubed chamber if the barrel has just been cleaned. The current "best way" to lube the carbine is to be very generous with the oil. Some of this gets on the ammo, especially if the gun is just lubed for a considerable time and not cleaned. Have you seen pictures of carbines lubed and not cleaned for thousands of rounds? They are an oily mess inside! Some of that gets on the brass.
As Melvin Johnson, the designer of the WW 2 Johnson Automatic weapons, stated, the extractor is greatly helped by lubing the cases and when the ammo is lubed "almost any old hook will do.". Ideally, your weapon should run well without lubing the ammo, and then for added reliability when it really matters put a very light coat of grease or wax on the case. In other words, lube your duty ammo.
Dave
INNOVATION IS SELDOM ACCOMPLISHED WITHOUT CONTROVERSY.
My first rule of a gunfight, thanks to John Farnam's wise advice. "Get away from there!"
Yes, typical practice in engineering. It would be interesting to know what kind of safety factor AR bolts are designed with. I agree that good design would take into account the possibility of lube coming into contact with the ammo or chamber.
If adhesion is a different phenomenon than friction, though, how is it measured or quantified? What are the units?
B.A.S. Mechanical Engineering Technology
Eric,
In addition to friction, there is the strength factor. Brass varies in thickness and can stretch accordingly. As an extreme example, a case that has been reloaded several times may have a ring of weakness in the body. The head may be ready to separate from the forward portion at any time. If it does that, there is almost no "friction" whatsoever. The designer of the weapon must design the locking system as if there was no case at all and still provide a large safety margin.
"Never put lube on the ammo" is just one of the conventional wisdoms found in the world of firearms. Conventional wisdoms are very often completely false.
Dave
INNOVATION IS SELDOM ACCOMPLISHED WITHOUT CONTROVERSY.
My first rule of a gunfight, thanks to John Farnam's wise advice. "Get away from there!"
Agreed, any competent firearms designer will design a bolt to have double-safe strength. That is, bolt thrust safety margin to deal with all the potential "no case friction" situations." And then a margin on top of that, simply as good engineering practice.
You want to keep lube off the case for more reasons than increased bolt thrust, such as lube attracting grit and other smut, cooked lube building up in the chamber to confound proper feeding and locking.
The operating envelope of the AR is narrow in many parameters. This is one of them; too little case adhesion/friction, and you increase bolt thrust, and accelerate wear. Too much, and you have extractors slipping off rims, tearing through them, or breaking.
Where we at with this? Anyone tried an E2 barrel? The idea makes sense and I doubt the four small flutes are enough to do anything to bolt life.
As an aside I believe the HK417 uses a fluted chamber.
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Last edited by Tokarev; 11-07-19 at 13:57.
“The rifle itself has no moral stature, since it has no will of its own. Naturally, it may be used by evil men for evil purposes, but there are more good men than evil, and while the latter cannot be persuaded to the path of righteousness by propaganda, they can certainly be corrected by good men with rifles."
Here is my dumb question for all of the bright people here.. Wouldn't the pressure applied to the case shoulder through the grooves be the same as everywhere else inside the case/barrel behind the projectile? What does pushing on the shoulder do that pushing on the inside base of the case isn't already doing?? I guess the grooves could equalize the pressure between the outside of the case neck and chamber to allow the brass to rebound away from the chamber neck before the extraction process begins.
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