21 December 2013

It's A Gas

Gas operated guns all work basically the same.

Some of the burning stuff what's pushin' the bullet is tapped to push the action.

You need two things in balance to make it work.

First the hole where the gas is tapped needs to be big enough to pass enough gas to make the action cycle.

Second the pressure that's feeding that work needs to be maintained long enough that the needed volume is tapped.  This is called the dwell time.  Dwell time is what matters most in an AR.

On most guns this balance is made by the designer and you don't have a choice in the matter.

The AR, on the other hand, has multiple barrel lengths and gas-port locations.

There are four gas port locations.  Pistol, Carbine, Mid-Length and Rifle.  We're gonna ignore pistol because we're talking rifles here.

Unless you're doing an SBR, your barrel choices are (effectively) 16" and 20".  There are some choices between and some longer, but 99% of the market is 16 or 20.

The 20" barrel is the original rifle length and, SURPRISE, normally comes with a rifle gas system.  This is the original balanced design and works well.

Way back in the early sixties, Colt decided that a shorter M16 would be handy, so they devised the R605.  The 605 is simply a standard M16 with the barrel cut down to the front sight base with just enough poking out to put the flash-hider on.  That makes it about 15-3/8" long.

It didn't work very well in cold weather because there's a pressure drop with cold ammo and there's just not enough dwell time to get the action going at those pressures.  Making the gas port larger doesn't always make things better.

Modern clones of R605 with 16" barrels don't fare any better.

Colt's next attempt to make a short M16 resulted in the R607.  The R607 introduces what's known as the carbine length gas system today.  It has a 10" barrel with the same dwell time as the R605 and since the gas port is much closer to the chamber it is tapping much higher pressure gas.  It turns out that it too doesn't have enough dwell time.  A 3.5" moderator was developed to take care of both the excessive muzzle blast and give more effective dwell time.

A 4.5" moderator was developed for the 10" barreled XM177 and XM177E1 to increase reliability and finally the barrel was increased to 11.5" in the XM177E2.

Eventually Colt looked hard at the problem and found that the ideal dwell time for the carbine gas port was a 14.1" barrel.  Since a 14.5" barrel is identical from the gas-port forward as a rifle, this was the length they settled on for all their carbines ever since.

Except...  14.5" is an SBR.  Commercial guns got the legally mandated 16" minimum barrel length.

A 16" barrel with a carbine gas system is actually over-gassed because it has too much dwell.  But it's generally good-enough and it was simple to do.

The next developments histories are a bit murky.

The idea of the R605 really snagged the imagination of many.  A rifle's sight radius with a shorter handier barrel seems ideal.  Thus was born the Dissipator (I think Bushmaster did it first).  The Dissipator is a 16" barrel with a front sight base in the rifle position, but with a gas port in the carbine position and hidden under rifle handguards.  It has the same over-gassed situation as any other 16" carbine.

Finally there's the mid-length gas system, which is literally halfway between the carbine and rifle gas port distance.  What it has is nearly the same dwell as the 20" and rifle or 14.5" and carbine but with the legally mandated 16" barrel.  Personally I think this is the ideal system for a 16" barrel.

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