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Discussion Starter · #1 ·
This is a bit long winded so stay with me :

Cheetahman posted the thread "bush gas system vs rifle" where we debating whether a few thousands (.001 to .002) of additional gas orifice increase in the bush barrel was really necessary to make the rifle cycle. I have some thoughts on this I would like everyone to think about. I have drawn some conclusions that may provoke others.

The Bush rifle is 40 1/2 in long and the Standard rifle is 44 1/3 in long meaning that about 4 in of the Bush barrel is gone. This leaves less time for combustion pressure to feed the gas piston.....SO-- :arrow:

#1 it is not reasonable to think that after subtracting about 2/3 of the barrel length (TIME) beyond the gas orifice, cycling of the weapon can be restored by opening the orifice by .001 or .002 because that would only increase the gas flow by 2%-5%. I'm saying that I am cutting the TIME to fill the pistons gas chamber 2/3 through a hole that is only 2% to %5 larger. That would be a extremely "hard sell" to anybody with a mechanical or engineering back ground. :!:

#2 adding a few thousands to the orifices diameter must be attempt to "Tweak" ie."Add a bit of insurance" to the system. The explanation as to why the orifice increase is so little leads me to consider other explanations as to why the gas system has ALMOST No Change :?

#3 Here are my thoughts---> The TIME need to fill the pistons gas chamber quickly enough to drive the cycle and push the piston past the waste gate vent is done in only about 2 inches past the barrels gas port. The rest of the barrel distance (TIME) is ineffective in influencing the gas pistons job because is over and done AND the excess entering gas is being exhausted out the bottom waste gate orifice.

#4 The gas system is being driven buy the STILL EXPANDING energy of the explosion and that ends beyond 2 in needed to drive the cycle.

When mechanics think about a problem they start at the end. Engineers start at the beginning, hopefully they meet in the middle...... I would invite your experiences and input :D
 

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Not being an enginmineer nor a michanic,and yes I can spell,if I wanted too.How far is the gas port from the end of a "bush/squad" 18" barrel.
If its 2 inches then isnt this all moot?And what's the size port on a "bush"
barrell,seems like there are alot of them out there with no problems.
UNLESS you're worried in a full auto, that might make a difference,yes?
BTW I'm a electrimigchian. 8) :D 8)

SEMPER FI
 

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Discussion Starter · #3 ·
Bush port is .080in (I think). When I first looked over the gas system I assumed that the entire length of the barrel was needed to make the thing cycle...... That it need that much time to fill the gas chamber. But when this deal about Bush Barrels came up I began a search for the "why" behind the "short barrel and almost no change" thing. Mechanics just think about this stuff just because its there :D

Ps don't worry about your spelling around here. I don't care if you spell stuff right or not. I enjoy your response.........Thanks
 

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Twiggy;
Miss spelled on pourpose,in house trades,I'm a sparky.
I wasn't concerened about the diameter,,,What's the distance from muzzle to gas port ,on a bush type 18" barrel?
If, when all is said and done,and you only need 2" past gas port.
I'm going to say that there is?2" past gas port on a 18" barrel?

SEMPER FI
 

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Air is an amazing thing at great pressures. Once it gets the chance to fill something it takes no time to do so. I'm sure some of you have used an air tank of some sort at some time and when you turn that nob on the top and it fills the guage unit it's done in nothing flat, just a quick hiss. It's the exact same effect. I don't even think it'd need two inches past the port to cycle, I'd say as soon as the edge of the bullet clears the port it's half way done cycling cause anything past that it's losing pressure. If anything the larger port size might be SA's way to 1) insure it gets done 2) keep the patent office busy. Just my take on it...
 

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Well I'm no engineer or mechanic either.....but I did sleep in....jest kiddin.
I guess the only real way of finding the point where the rifle cycles can only be done by cutting small pieces off a barrel until it doesn't cycle anymore....anybody got a barrel they wanna donate for the experiment????
 

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The good folk at FALFILES have exhausted this one alot. Find someone to search there. As I remember, carbine port size was .118 but to my mind that is too large so memory might be off. Standard Fal port .095 Hard to even know if the memory is going or has gone yesterday.
My say is that the pressure is quick, there is a chamber in front of the cyl. to load up. That pressure isn't going back into the Bbl, it will work on piston. As gas piston moves the chamber gets many times larger making 'fit' of piston ie. no leak important. So bottom line my guess is .005 to .010" to make the carbine work. Let us know.

Edit: I did the FALFILES search Well known smith there says for a 17" bbl (on FAL) port size .112-.116MAX
Others said new DSA 13"bbl. port dia.115"
Still another thread said 16"= .110 to .120"
18"= .105 to .110 7/64=.109
Just for Ref.;; thought this might give ballpark help.
 

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What I've taken from all this discussion and informative posts starting out over on that other forum, is this: The M14 rifle design is a combat design. The last thing you ever want is for some enviromental snafu messing up your rifle function. Therefore I now believe that the rifle's gas system has a huge window of function, most of which is not needed under normal circumstance. However it must still function when sandy, muddy, wet, frozen, much fired and uncleaned for a month, etc. The design uses only what gas is required to function and vents away the rest. But I would bet that the scout barrel design is probably much less forgiving. For instance has the shorter design ever seen any significant combat experience. Perhaps someone here can say. I don't think SWAT duty or even DEA enforcement activities would subject the rifle to the same worst of the worst conditions that infantry service would subject the weapon to. Just my opinion.
 

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Discussion Starter · #9 ·
Your probubly right. All the time I was in the service I never saw a Bush rifle :D
 
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