Difference between an AR-15 or M4 carbine - Page 2
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Old 05-02-2013, 06:53 AM   #11
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I've heard the insert must be black in military M4s. Blue is acceptable in the M16 but they're being replaced as a wear item with the black insert and corresponding spring. O-rings are preferred in the M4 but many of us remove them from our semiauto versions.

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Old 05-02-2013, 09:59 AM   #12
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So what is the function of this popularly-debated o ring/buffer/whatever it is? Why does the military care what color an unseen part is?

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Old 05-02-2013, 12:21 PM   #13
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It aids in extraction in overgassed guns (most carbine length gas systems) and with the m4 14.5" barreled gas system.

What happens is the gas system in a carbine length is so short and often oversized in longer barrels the case hasnt had enough time to shrink after firing before the gun starts extracting. The black oring is a brute force solution to design problem with short gas tubes and short barrels.

Midlength gas systems solve this issue in 16" barrels hence their populaity

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Old 05-02-2013, 02:36 PM   #14
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Originally Posted by clr8ter View Post
So what is the function of this popularly-debated o ring/buffer/whatever it is? Why does the military care what color an unseen part is?
The color of the unseen part is to color code it for how firm it is. Blue is for rifles, Black is for carbines. I've seen both in both.
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Old 05-03-2013, 04:59 AM   #15
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A Colt model LE6921 is the closest thing an individual can, since 1986, legally purchase to a US military M4 carbine. The only difference between the LE6921 and the M4 carbine are the fire control parts and the lack of a sear pin hole in the receiver.

The extractor "o-ring" is an inexpensive solution to a problem that should have been resolved with higher quality extractor springs. The military is always looking for new and inventive ways to save a buck or two, which is the reason behind the blue and black o-rings.

The 16" barrels with carbine length gas systems have two things going against them:

1. Dwell time - After extensive testing the military determined that having 4 inches of barrel in front of the gas port was about optimal. The M16 rifles and M4 carbines have that magic 4 inches of barrel in front of their gas ports. The 16" barreled carbines with carbine length gas systems have an extra 1.5", roughly, in front of the gas port. The "mid-length" AR's locate the gas port further forward to make up the difference.

2. Location of the gas port - It shouldn't be a big surprise that the closer the gas port is to the chamber, the higher the pressure in the barrel will be.

As JonM noted, most AR's to include military AR's, use more gas than necessary to forcefully operate the action of the rifle or carbine. The result is greater wear and tear on the operating components and shortened service lives. Automatic, burst and fully automatic, modes of operation cause problems not generally experienced by civilian AR users (like extraction failures, bolt bounce, overriding cartridges from magazines with weak springs, and a number of other not-so-great side-effects).

Unless you're a government user you probably won't have to worry about the auto problems, but you may have extraction problems from worn, damaged, or out-of-spec components. The best answer to the extraction problem is a better spring, not rubber or plastic doo-dads.

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Old 05-03-2013, 10:17 AM   #16
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Verrrry interesting. Now I'm curious as to what color is in mine. But regarding the spring KBD512, is the spring you speak of available as a fix for the problem?

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Old 05-03-2013, 07:16 PM   #17
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Verrrry interesting. Now I'm curious as to what color is in mine. But regarding the spring KBD512, is the spring you speak of available as a fix for the problem?
I'm not KBD but BCM sells an extractor upgrade kit with the proper spring and black insert. I have three on hand since they are small and cheap. Good idea to order a few spares.
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Old 05-03-2013, 10:05 PM   #18
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Cool, thanks.

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Old 05-04-2013, 01:22 AM   #19
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clr8ter,

I'm speaking of chrome silicon extractor springs from manufacturers like David Tubb (davidtubb.com) and Sprinco (sprinco.com). I'm sure there are others, but those two immediately jump to mind and are things that I use.

The materials that most manufacturers use and many military specifications denote are about cost/economy, not about the best performing components for the job.

With a proper chrome silicon extractor spring you don't need little rubber parts to make your extractor grip the case rim with enough force to assure a reliable extraction. In fact, Tubb tells you not to use a D ring with his extractor springs. I'm not sure about Sprinco, but I'd bet that they'd tell you the same thing.

The point is, better materials technology and better manufacturing methods produce springs that function better and longer without replacement. The same is true of the buffer spring.

With the number of compressions and level of heat your valve springs are subjected to, ever wonder why your car can go so many tens of thousands of miles before those components wear to the point that they have to be replaced? Ever wonder why, if that's possible in an internal combustion engine, it would be necessary to replace a buffer spring or extractor spring after a few thousand rounds?

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Old 05-04-2013, 02:53 PM   #20
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Oh, no, i'm aware of that, I don't wonder. I am new to Ar's, though, so I'm not fully up on what all the parts do, and if there are better replacements out there. Thanks for the info.

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