I talked with a gunsmith with some experience in prototype weapons and he liked some of my ideas. One thing I hadn't thought of that he did is that it will lend itself well to a "continuously variable" cycle rate. Big heat and inertia problems though. Apparently, energy can't be created or destroyed, only change form. Damn! What percentage of heat in a bbl. comes from friction vs. hot gas?
Machine gun update
May 05, 2004
6 Replies
Never having studied specific thermodynamics of the machine gun, I would liken it to any other IC (heat) engine. Oh, Wait! I've not gone into that thorn bush very deeply either. Sorry.
I think it is virtually all hot gas. If you hand push a bullet through a barrel the barrel does not get very hot, & classical physics IIRC would tell you that the velocity of the travel does not matter.
You might want to get a book called "Hatcher's Notebook." Gen. Julian Hatcher, the author, participated in or managed much of the small arms development for the army from WWI until WWII (approximately, my copy is
200 miles away). He was also a popular author and an NRA official into the 1950's. He treats machine guns quite extensively and there is a lot of other very interesting stuff in there. For example, I recall a discussion about the problem of a live round getting stuck in a nearly-red-hot MG chamber and how to get it out before it cooked off. Or worse, having it cook off as you were getting it pulled out.Geo. Anderson
Don't know for a fact one way or the other but I can tell you that it is easy to make the gas cylinder on an M60 glow cherry red from shooting blank ammo so my money would be that most of the heat is from the gas.
Steve.
I echo Geo on Hatchers notebook.
I also suggest a water jacket.
Gunner
"A vote for Kerry is a de facto vote for bin Laden." Strider
Good question with no fixed answer. The logic goes like this: Energy in before firing is the chemical energy of the explosive. Energy out of the barrel after firing is the KE of the bullet (1/2mv*2) + energy contained in the hot gasses that leave the barrel. The rest is absorbed by the barrel. Energy before = energy afterwards. All you have to do is identify where it is and quantify it. Don't be fooled by the lack of friction of a bullet in the barrel. A bullet deforms under pressure and swells to fill the barrel. This causes it to engage the rifling and seal better.
John
What percentage of heat in a bbl.
I agree. I read an interview with one of the developers of the M16. He was real clear that the chief source of heat is from the friction of the bullet passing down the barrel. Although a machine gun fires say 8 to 20 rounds/second, the amount of time the barrel is exposed to hot gases is very small as the bullet accelerates down the 6" to 20" barrel to 1000-3000 feet per sec, and heat transfer from gas to metal is not great. I think the gas tubes on gas operated mechanisms see a lot of heat because they both trap gas and the gas does the work of operating the gun's feed mechanism. I can't remember my thermodynamics well enough, but I suspect the work results in heat build-up as well.
Fred
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