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Discussion Starter · #1 ·
Does anyone know if there are any modifications neccessary to the gas system when changing to the 18 inch barrel? It seems to me that there would be a definate difference in the duration of the gas pulse available to operate the action, given the 4 inch difference in barrel length. It may be that the volume of gas and the pressure are still well within the operating envelope. I ask because I'm gathering some parts for a M1a bush rifle build up. Years and years ago, I built an SKS carbine from a kit, and I had definate shortage of gas to operate the action, after cutting off some barrel. Thanks guys in advance.
 

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Aloha Cheetahman:

I don't think that there should any change to the gas cylinder system... even though the barrel itself will be shorter, the location of the gas system doesn't change, so I would think that up to that point it should be identical to a standard rifle. At least in terms of gas volume.

I apologize in advance if this information and opinion is in error, I do not have a Bush style rifle but definitely looking into it...

Aloha,

Tom O.
 

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Discussion Starter · #4 ·
I try to think about this problem in this way. The gas pulse to operate the piston has a duration of about the time it takes for the bullet to travel from just past the gas port in the barrel, until several milliseconds after the bullet exits the muzzle and the pressure drops. In a normal barrel this distance is about 6 inches. At 2500 fps it takes the bullet about 1/5000 of a second to travel that entire distance, all the while feeding gas into the port. With 4 inches off the barrel, this time is shortened to about 1/15,000 of a second, or 1/3 the time and therefor 1/3 the energy available to operate the action. My bet is that they(barrel makers) enlarge the gas hole a little in the barrel, and then the assemblers chop a coil or two off the main spring, to get things to work. However I just don't know so I'm asking.
 

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Sorry got to remember to log in. That's why everyone says heavy bullets hammer the tar out of your action. They are just plain traveling slower, so the gas pulse is stronger and the bolt slams back harder. If you use Shusters nuts which vent some gas I don't think a heavier bullet would recoil any heavier than the 150's. At least not from the action's point of view.
 

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I had a second to jump in here.I just can't stay away I guess.

The "only difference" in the gas system between the bush and standard is the size of the gas port.Bush barrel is slightly larger by about .001-.002

HTH-warbird
 

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I'v heard that a NATO charge burns completly in 18 1/2 inches of barrel. Of course, the only thing that matters is that How Much Past the port do you have to do the work. If the guys with the vented plugs are doing OK with standard barrels then probubly your OK just the way you are. A thought on Enlarging the gas hole-- Doubling the size means Four times the flow, so a few thousands means much more flow even though it seems trivial.
 

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Twiggy said:
I'v heard that a NATO charge burns completly in 18 1/2 inches of barrel. Of course, the only thing that matters is that How Much Past the port do you have to do the work. If the guys with the vented plugs are doing OK with standard barrels then probubly your OK just the way you are. A thought on Enlarging the gas hole-- Doubling the size means Four times the flow, so a few thousands means much more flow even though it seems trivial.

home at last, home at last.

Here's how it works.
All mil-spec amd most standard caliber ammo uses a slow burning powder.Exceptions being magnum and wildcats which use a medium to fast burning powder because of case capacity.

M14's,M1A's amd M1's are port pressure sensitive.
Now,chamber pressure is determined by bullet weight and barrel length.If you increase bullet weight or barrel length,chamber pressure goes up because the bullet stays in the barrel longer..Port pressure also goes up (at the gas port).

Here's the easy part.If you shorten the lentgh of the barrel from 22" to 18-1/2",chamber and port pressure is reduced.To compensate for reduced port pressure,you must increase the gas port hole to increase port pressure at the gas system so the action cycles the same.

If you install a bush barrel,the gas port hole should already be the correct size.However,on some of the older bush barrels from Sprfld,I've had to increase the size of the gas port because somebody had drilled them the same as a standard barrel by mistake.Yeah,I checked with Spfld at the time to make sure before screwing up (I always do my homework).

By the way,the gas port hole on a USGI barrel is .078
Remember,I said only .001-.002 difference "not a few thousands.
Also,all powders used in standard calibers burn completely within 18-1/2" of the barrel.Of course,there are exceptions to the rule like handloads,short magnums,etc.

I hope ya understand all this mumbo jumbo. :roll:

Now it's time to count sheep. :D

warbird out
 

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I also realize now that according to warbird's post, a heavier bullet weight would most likely cause higher chamber pressure which would upon ignition, cause more force upon the lock up lugs of the bolt and since that force is transferred to the receiver, more overall stress on the receiver. I don't think there is much that could be done about that. It's just natural laws. But I do think that a gas venting device like the Schuster nuts could help take some violence out of the action cycling and help reduce stress on the op rod. If I'm out in left field, I hope someone will chime in with reality. PS I have nothing to do with Schuster or his nuts. Thanks Mike
 

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Cheetahman said:
I also realize now that according to warbird's post, a heavier bullet weight would most likely cause higher chamber pressure which would upon ignition, cause more force upon the lock up lugs of the bolt and since that force is transferred to the receiver, more overall stress on the receiver. I don't think there is much that could be done about that. It's just natural laws. But I do think that a gas venting device like the Schuster nuts could help take some violence out of the action cycling and help reduce stress on the op rod. If I'm out in left field, I hope someone will chime in with reality. PS I have nothing to do with Schuster or his nuts. Thanks Mike
Now you gettin it. :D


warbird out
 

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Warbird,
Your post made me think of this, and I don't know why, but I was wondering if you might have an answer?
Has anyone ever made a rifle with a fluted chamber?

Regards

Ox
 

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HTH
Isn't there a fluted chamber on the H&K 91(I hope, it's the .308 version) or do you mean the M14/M1A type? A friend has an H&K and it leaves lines/striations on the brass, you can hardly feel them, H&K says it makes for easier extraction and he shoots any and all kinds of .308/7.62. He doesn't reload so he leaves the brass at the range, he said no one wants it, because of the lines on the fired cases. The belief is they are to hard to clean and resize. I'm sure there are some who have one, is this the case or another urban legend?
 

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You're correct tatonka.
H&K also uses fluted chambers in the 93,94 & PPSG (I think,the sniper rifle).I've worked on many of them.

Fluted chambers were also used in some 22 magnums like the AMT 22 mag automag.

The reason for fluted chambers is because it delays case extraction which enhances accuracy much like increasing bolt lock up time.Fluted chambers slows down the cycle rate which was the problem on the automag untill they fluted the chamber to fix that problem.


warbird out
 

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I was at dinner tonight with my brother and we were kicking around the shorter barrel theory that is being posed here. Mark is a machinist and what they call a piratical engineer. When an engineer presents his department with a item he calculates if it can be built and the probability if how it will work in the "real world".
I am a certified mechanic and deal a lot with fuel injection flows, orifices, pressure etc. So we have a lot of fun with these types of discussions. ANYWAY: the rule of the thumb in orifice flows is that the "window" of function for a orifice should be 10% in other words if orifice in a barrel is .078 there should be built into that size the idea that things may change because of Residue restriction, pressure changes, etc. and that the rifle should still function at least to the point that the FLOW is changed 10%. In the case of increasing the size of the orifice .001 or .002, that will not change the flow of the orifice 10%. What we are saying is that if .078 will work, .080 will also work. That is only 4.93%. SO: even though M14 gas orifices may be considered sensitive they appear to be working within a rather normal paramater.
 
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