Is this true?  I would think that you would have the most push on the bolt as you have the longest length of barrel past the gas block of any of the variants.

As I understand it the short gas tube guns are the least reliable.  The mid-length is better, and the 20" is the most reliable.   IIRC the Marines tested the M4 against the M16A4 and found the M16A4 to be more reliable.  I don't know if the extra 1.5" of barrel out front would help or hurt the reliability vs. the 14.5" barrel, but since the failures are due the the harder cycling caused by tapping the gas too close to chamber where the pressure is high I don't suppose it would matter much.

Ak's are even harder cycling, and they are more reliable.  I would think that unless you had a weakness in another part of the system, more vigorous cycling would = better reliability?

I don't know much about AR's, so I'm curious.  Maybe as you said, the shorter gas system far outweighs the extra energy available for long term reliability.

I know there's a delete button around here somewhere.

Quoted: Ak's are even harder cycling, and they are more reliable.  I would think that unless you had a weakness in another part of the system, more vigorous cycling would = better reliability? .

Since the gas systems are completely different, your comparison doesn't work...

The 20" is widely held to be the most reliable format.

20" definetly.

Although I've never had a gas related problem with 50+K rounds with my 16s or 14.5.

I have gas somtimes... Oh, sorry... wrong thread...

The bbl. length effects terminal ballistics. The position of the gas port effects cycling & reliability.

With the port closer to the chamber, the propellant gas impacts the carrier key sooner, causing the bolt+carrier to cycle sooner & faster. The problem here is that this time reduction doesn't allow the case to relax from its initial pressure-formed fit immediately after firing. The tighter fit between the case & chamber puts more stress on the extactor during extraction, causing occasional failures to extract due to insufficient grip on the case to pull it from chamber, & increased fatigue & eventual failure of the extractor itself. The problem seems to be more of an issue on FA firing, less w/ semi's.

The basic cause is the reduced gas dwell time in the tube, & the solution is to increase its volume. Whether by increasing the port distance & using a longer tube, or by other methods, like PRI's fat boy (larger diameter tube of same length as the stock carbine tube), & the pigtail (coiled gas tube, wrapped around the bbl.) the increased volume gives the case more time to relax & cooperate w/ the extractor.

as a rule, the farther you get from the original design, the less reliable a device is gonna be.  it doesn't make economic sense to reengineer something for a slight modification like barrel or gas tube length.  they usually just change the springs enough to make 'em work and sell it.  i'm just pissed i couldn't use a cool term like dwell time....    

It has been my experience that the 20" barrel is the smoothest, most reliable operating, followed by the 16" with the midlength hanguard (RRA and Armalite).  

The Carbines generate about twice as much pressure at the gasport as the 20".  But the AR-15/M16 design is, to a small extent, self regulating.  No matter, the short, high pressure pulse of the carbine is more of a balancing act as far as spring rates, etc.  They sometimes suffer from too much pressure.  

Some people have experienced malfunctions with AR carbines and the first thing they did was drill out the gas port.  This shoots even more pressure down the tube, and their problem got worse.

At one time, most carbines on the market were built with surplus 11.5" military barrels with welded on 5" flash hiders.  The gas port on these was sized to work correctly.  Then when they started to make true 16" barrels some mfgs made the gas port the same size, or they just did not get it right for whatever reason.  The bolt carrier actually went back too fast, bounced off the back of the buffer tube, and started back forward too soon.  This was felt as excessive recoil.  Then the ejected brass did not have time to clear the ejection port and this resulted in a stovepipe (the ejected round) with another round pushed up under it halfway into the chamber.

So guys thought, "oh, it must not be getting enough pressure, drill out the gas port." And the problem became worse.

For the last 12-14 years the mfg's seem to have gotten it right, and most 16" AR's run pretty smoothly.

Great info Thanks.

5oz tunstien buffer's slow that cycling down a little...  as yojimbo pointed out to me it's extra mass slows down the initial cycle down to allow pressures to lower...  he says it reduces recoil aswell...

also colt uses the high power extractor springs to increase reliablity...

...

yes 16"ers are sharper for the most part... but you can make steps to correct it...