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Old 01-21-2010, 00:29   #1
Petrie
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Length of gas system?

I actually stayed away from M4 style ARs because I had read that they extracted more violently. My understanding was that the M4 has a barrel length of 14.5" and that the gas system is 7" long and this works great. But, when ARs get made for the civilian market manufactures use a 16" barrel, but still have a 7" long gas system. So, what happens is the bullet is still in the barrel as the gas is hitting the gas key and opening the bolt. So, the pressure of the gas in the barrel violently blows the bolt carrier back. Which means the extractor spring and the components get the piss beat out of them, or something to that effect. Am I correct? Also I read that a midlength gas system is 9" long and works the best for a 16" barrel.
Basically my understanding is that the bullet has to be out of the barrel so the pressure in the barrel drops just before the action opens, otherwise the bcg is going to be blown back violently. I appreciate any comments
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Old 01-21-2010, 12:00   #2
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The short version: Carbine length gas for 14.5" barrels, midlength for 16".

You're 90% correct. The 7" gas system is harder on the gun (and extraction) because the gas 7" down the barrel is a lot hotter and higher pressure than a standard rifle length system. In fact it's roughly twice the pressure. Having a carbine length gas system on a 16" barrel increases the dwell time, or basically the length of the gas pulse, which is even harder. A mid length gas system on a 16" barrel puts the dwell back where it should be and reduces the temperature and pressure of the gas hitting the port. This aids extraction because the harder you slam the carrier back the more likely the extractor is to slide over the cartridge rim. The gas is hitting the port with the bullet still in the barrel no matter what, even on full length systems.
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Old 01-21-2010, 12:57   #3
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Old 01-21-2010, 13:54   #4
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A CAR gas system on a 16" barrel is rougher than on a 14.5" barrel, but only slightly. A CAR gas on a 14.5" barrel is already pretty rough. In combat, the M4 has experienced many more malfunctions than the M16. The band-aids for this have been extractor O-rings and heavier buffers, but this is simply a matter of treating the symptoms.

Ideally:

11.5" and under get a CAR gas system.

14.5" - 16" get a mid length gas system. (More and more manufacturers are beginning to put mid gas systems on 14.5" barrels).

20"+ gets a rifle gas system.

18" barrels work well with either mid length or rifle length.
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Old 01-21-2010, 13:56   #5
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Double post.
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Old 01-21-2010, 14:01   #6
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Triple post, apparently.
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Old 01-21-2010, 14:14   #7
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So, should a guy just stay away from an m4 style AR? I was looking at getting this upper:

http://www.dpmsinc.com/store/products/?prod=1770

I see some of there 16" rifles have handguard that goes right up to the front sight. So, they must have a rifle length gas system on them right?

Also the Gernade launcher cutout on the barrel seems like a waste.
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Old 01-21-2010, 14:45   #8
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Good post. This was something I have heard of but never really got any definite info. I guess you must consider the use of your particular AR. Military and competition shooters are alot more hard on their guns than most civilians, even police. I would classify myself as a recreational shooter and don't shoot more than 1000rds a year. Would if be safe to assume that for the majority of casual shooters with 16 inch guns, the gas system shouldn't be that big of a concern?
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Old 01-21-2010, 16:53   #9
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Wow!
Thanks guys.
I'm looking at the AR platform those days myself.
Much appreciate the info.
I'm kind tore between 20" rifle from Stag and 14,5" carbine (not 16") from Norinco
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Old 01-21-2010, 17:36   #10
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A heavier buffer helps too.

I love 14.5" carbines. They are still very reliable.
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Old 01-21-2010, 18:24   #11
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IMO you guys are reading way too much into this.

If I didn't know anything about AR's and I read this post I would think a 16" with CAR length gas system was going to to beat itself to death in no time.

In reality there's millions of them out there that are great rifles.

Last edited by internal; 01-21-2010 at 22:06..
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Old 01-21-2010, 19:33   #12
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Quote:
Originally Posted by internal View Post
IMO you guys are reading way too much into this.

If I didn't know anything AR's and I read this post I would think a 16" with CAR length gas system was going to to beat itself to death in no time.

In reality there's millions of them out there that are great rifles.
Exactly. I've taken carbines through carbine classes and thousands of rounds w/o a single failure of any kind.
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Old 01-21-2010, 20:38   #13
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Quote:
Originally Posted by Petrie View Post
So, what happens is the bullet is still in the barrel as the gas is hitting the gas key and opening the bolt. So, the pressure of the gas in the barrel violently blows the bolt carrier back. Which means the extractor spring and the components get the piss beat out of them, or something to that effect. Am I correct?
Not the way you describe it.

Trust, me. The bullet is long gone by the time the bolt unlocks. Any residual pressure in the barrel, if any, is not responsible for any battering of the action. mkmckinley has the explanation.
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Old 01-21-2010, 21:45   #14
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Quote:
Originally Posted by internal View Post
IMO you guys are reading way too much into this.

If I didn't know anything AR's and I read this post I would think a 16" with CAR length gas system was going to to beat itself to death in no time.

In reality there's millions of them out there that are great rifles.
Yeah I figured I was concerned over nothing. When I talked to DPMS tech today he said they use the same extractor set-up for rifles and carbines, but that on the carbines the buffer and return spring are heavier(or something like that). However, I have read posts that talk about faster gas port erosion with carbine length gas systems because the gas is hotter and under more pressure the closer it is to the action. Like you said, there are millions of Carbine gas systems out there so there is no real problem.
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Old 01-21-2010, 22:09   #15
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Quote:
Originally Posted by Petrie View Post
Yeah I figured I was concerned over nothing. When I talked to DPMS tech today he said they use the same extractor set-up for rifles and carbines, but that on the carbines the buffer and return spring are heavier(or something like that). However, I have read posts that talk about faster gas port erosion with carbine length gas systems because the gas is hotter and under more pressure the closer it is to the action. Like you said, there are millions of Carbine gas systems out there so there is no real problem.
No matter what gas system is used, even the barrel type (standard button rifling or cold hammer forged) the throat of the barrel itself is the part that's going to take the most beating/damage and no gas system and barrel design is going to change that.
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Old 01-22-2010, 02:04   #16
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Quote:
Originally Posted by DLEE25 View Post
Good post. This was something I have heard of but never really got any definite info. I guess you must consider the use of your particular AR. Military and competition shooters are alot more hard on their guns than most civilians, even police. I would classify myself as a recreational shooter and don't shoot more than 1000rds a year. Would if be safe to assume that for the majority of casual shooters with 16 inch guns, the gas system shouldn't be that big of a concern?
Yes. Especially if you use an H2 or H3 buffer. This effectively adds to the moving mass of the bolt group. Drop in an extractor O-ring insert, and you're fine.

When we talk about one gas system being better than the other, we're talking about maybe an 8,000 round service life out of a bolt instead of 5,000 or 6,000, or slightly better reliability under really bad conditions.

FWIW, there are plenty of guys "in the know" who shoot CAR gas guns day in and day out. I'm just super anal about my guns.
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Old 01-22-2010, 02:07   #17
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Quote:
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No matter what gas system is used, even the barrel type (standard button rifling or cold hammer forged) the throat of the barrel itself is the part that's going to take the most beating/damage and no gas system and barrel design is going to change that.
Even on a rifle gas system, you'll probably see a bolt failure (sheared lug or cracked in half at the cam pin hole) before you'll actually shoot a barrel out. Assuming you're not expecting super-match-grade accuracy, and AR barrel is good for 20,000+ rounds.
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Old 01-22-2010, 02:38   #18
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The best way to mitigate the risk of parts failure from short gas is to just buy good quality parts. We have many many rounds through our Army issue Colt M4's and while bolts do fail it's pretty rare. I think Ive seen it twice in 5 years. It usually happens during a SFAUC or something which is like our own version of a carbine course that lasts about three weeks. Our armorers don't observe any standardized service life, when they break they break so the ones I've seen break might have 20,000 rounds through them. I know. Every time I suggest having a training upper and a deployment upper I get laughed at.

What your looking for are bolts that have been used to fire an overpressure proof cartridge and then magnetic particle tested. You'll see a little MP etched on the bolt if it's had this done. Only a relative few manufacturers do this including Colt, Noveske, Daniel Defense, and Bravo Company. Ones that don't are Bushmaster, Armalight, and DPMS. Look for "the list" on M4carbine.net or ar15.com for a more comprehensive, well, list. If you have a non-MP bolt just shoot it till it breaks and get an MP one from Bravo Company. Another thing that can help, besides getting a heavier buffer (they come in four weights, CAR, H, H2, H3) is to use a full auto carrier which adds a little more mass. I also read a pretty technical article from on of the manufacturers that had evidence of corrosion leading to stress fractures that ended in bolt failure, so keep CLP on your BCG.

If you're going for a complete upper the same companies that MP their bolts do the same for their barrels and might even cold hammer forge them for a perhaps little more life. I think the Daniel Defense and Bravo Company uppers are the best thing going right now in the AR market with Noveske being sort of the Cadillac of AR's.
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Old 01-22-2010, 18:14   #19
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If the bolt cracks on a rifle/carbine and you buy a new one do you have to take it to a gunsmith and have him mate it too the lugs in the chamber? Or will a new bolt just wear into an already wore chamber lug?
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Old 01-22-2010, 19:12   #20
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If extraction is worrying you, consider Bravo Company's 'extractor kit upgrade' - $5 for an extra-heavy duty extractor spring/o-ring/rubber insert thingy ($12 for a 3-pack, if you have multiple bolts ar want a back-up).

To indicate the focus of consideration, I wonder how many would agree that most threads discussing this issue center around smoothing out the action (by using a heavier buffer, like Spike's Tactical's product (ST2 or something), and using a mid-length gas systme on a 16" AR barrel), rather than saving the components from wear or risk of failure. This is an indication of the relatively moderate effect these two items would have in the proposed application. It's more about 'making a good thing 'better'' than it is about 'solving a 'problem''. I have no technical data to offer, nor any expertise on the previously provided explanations - this is just my take on the many thread conversations I've read regarding these two components.
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