AC, I saw M-60's cook off a number of times in Viet Nam. Door gunners sometimes tended to stay on the trigger and when things got hot there would be a very brief pause after they stopped firing, then "blam", a pause and another "blam". They usually would break the belt after the first or second shot. The guns did NOT continue like they were in full auto. As I recall it was 2-5 seconds to cook them off, and this is vague but I'm guessing about a 30 second sustained burst, or something like that to promote such an occurance. It is notable that we carried an extra barrel too, and ASBESTOS MITTS were SOP for a barrel change.

Folks that flew recon in the Air Cav units in Nam as Aeroscout crewmembers saw more 7.62 and 5.56 ammo fired in a year than most gunnery instructors would fire in a lifetime. I saw a lot of curious things in 2.5 years over there, the above being one of them.

I do not know if infantry people experienced the same thing or not. Their ammo supplies were much more limited, and they tend to fire short and infrequent bursts. On the down side, we had more air cooling than they did. And we carried 1500-2000 rds on a mission for the -60, as opposed to the 100 rd. belt around the chest which was a common acoutrement during that war for infantrymen.

Ricochet...

As nearly as I can tell, that's pretty much how Q applies to a barrel. If that's true, and I think it is, you ought to be able to reduce Q by putting damping material in the barrel channel. Browning does do that.

The criticalness of the exit point is probably more controlled by the resonant frequency of the barrel than by the Q, but, in general, I think we are in violent agreement.

Harold Vaughn has a chapter in his book about the whole forend pressure thing, which kinda blew me away. His theory is that it is very hard to make a really rigid threaded joint. Part of the problem is that the first thread carries about a third of the load, and the threads farther out carry only a very small part. He recommends a special thread, called a SpiraLock that makes a much more rigid joint. He says that forend pressure pre-loads the threaded joint, and reduces variability. He also says that when you screw a new barrel into place, you should wrap the threads with Teflon tape, if you can't get the Spiralock thread. I had simply never thought of the issue before.

It makes perfect sense that forened pressure would also damp barrel oscillations, especially if the pressure was located far from a node.

Harold's book is a kick. Here's a guy with an old tube type Tektronix 555 oscilloscope, which he carried around in the back of a pickup, with a gas generator. He instrumented guns with strain gages, and measured a bunch of stuff that a lot of us pay no attention to, such as how much the backward force on the recoil lug lifts the tip of the muzzle. The only thing I don't like about it is that he left the planet before I got a chance to know him, and ask him some of my yet unanswered questions.

Anyway, more mass in the barrel would lead to a lower resonant frequency, and greater stiffness would lead to a higher resonant frequency.

AC, I find it very easy to believe that the standard deviation of MV has a lot less effect on accuracy that barrel vibrations do... So how do you successfully damp a barrel? If we can figure that one out, we can be famous, as long as the solution does not involve something like 200 pounds of concrete at the muzzle.

Ricochet,

Excellent post about harmonics. Equipped with triaxial accelerometers, one can determine barrel harmonics and damp accordingly. Modelling a barrels harmonics is difficult at best. A colleague of mine has discovered that barrel harmonics has FAR more impact on the size of his groups than Standard Deviation of bullet velocity. Imagine that!

ASS_CLOWN

Hot Core,

Excellent point:

Quote:

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"A SGS can give excellent Pressure Data when "properly set-up" inside a Ballistics Lab that has a CMM to obtain the proper dimensional measurement accuracy for the Chamber wall

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The axial position is also critical to correct set-up of the SGS system, to go along with your correct statement regarding chamber diameter!

Setting up a strain gage system correctly is time consuming and NOT easy. All that work for something that still will only read accurately to the nearest 1000 psi.

This last sentence is my primary point.

ASS_CLOWN