But you are correct in your assertion that it is not impossible. Nothing is impossible. I tend to speak in terms of absolutes as a habit from arguing political philosophy, which follows a different set of rules from the temporal realm, to be sure. There’s also the Gatling system, which conceivably does not pull the fired case from the chamber until the pressure has become ambient.

I read once that David Marshal Williams of M1 Carbine fame once created a blow-forward operated rifle just because someone argued with him that it could not be done.

http://www.youtube.com/watch?v=wSfVJNEiQw4

I suspect this is not correct; maybe not easy to do but not necessarily impossible.
One possibility:
A spring is positioned between the gas piston and the bolt, latches are placed on both sides of the spring.
The latch on the action side of the spring is connected to barrel and high pressure gas will prevent the spring from moving; release of the latch will allow the spring to move toward the bolt.
When the rifle is fired, the gas piston will compress the spring; the second latch will engage automatically as the spring is compressed. The purpose of the second latch is to prevent the spring from moving toward the piston when the gas pressure is lowered. The second latch is released by the bolt moving forward.
Sequence:
1. Ignition
2. Bullet passes first latch port and latch stops spring movement toward bolt.
3. Bullet passes piston port and piston compresses spring against first latch. The latch on the piston side prevents the spring from decompressing.
4. Bullet leaves barrel.
5. Pressure reduced to a level that allows first latch to release and spring pushes bolt to the rear.
6. Bolt moves toward chamber which releases the latch on the piston side of the spring.
7. Repeat as necessary.

My arse would be twitching putting it away overnight without a cleaning, expecting a sergeant’s boot to connect at any time!

Based on my somewhat limited understanding of fluid dynamics, the pressure and gases which cause fouling will continue out of any available opening, following the path of least resistance.

There is no system which can operate the action based on the higher pressures within a firearm that will be able to utilize all available gasses without any waste, and no system of vents, valves, or pressure releving cells that could change the higher than atmospheric pressure inside of the bore to the ambient atmospheric level before that action was able to cycle.

When the action cycles, it breaks the seal created by the fired cartridge case. The barometric pressure inside of the receiver and action behind the breech will be equal to that of ambient barometric pressure of the surrounding environment, and when that seal is broken, those gases will travel backwards into the action at the same rate they will escape any other place in the system. You could probably calculate the pressure of the gases which escape into the action and pressurize the inside of the action with Nitrogen or some other inert gas to counter that back pressure, but that would be very impractical, even in suppressed machine guns.

By the time the action has moved any significant amount, the bullet has exited the barrel, and the gas pressure has already begun to vent out of the muzzle. The shorter the barrel, the less time this takes. But when a suppressor is used, the entire purpose of is to act as a hindrance to the natural flow of the gases, to slow them down and turn the movement of gaseous particles, which is observed in the environment as sound, and turn it into heat, by forcing those gases through a series of baffles. A suppressor could theoretically be too efficient, and block so much gas that would otherwise escape out the muzzle that when the action opened the effect would be the same as when an over-pressure round causes a case head separation. Boom!

Suppressors slow the release of gases. If the weapon uses those same gases to operate its action, then some of those gases will escape into the action. There is no way to capture that energy and utilize it after that energy has been transformed into heat or escaped into the atmosphere without the use of electronically controlled motors and power cells. There will be blowback, and suppressors will make that blowback worse.

Anyone else what to chime in?

Not all firearms. I know of the Swiss SG 550 series of rifles has a value to allow only the needed amount of gas through. I wonder if these will run as dirty?

“Once inside the gas cylinder, propellant gases pass through an L-shaped channel machined in the piston head and are directed forward towards the gas valve. The pressure build-up in front of the piston head pushes the piston and bolt carrier rearward. As the piston is driven back, the gas port and the L-shaped channel move out of alignment, cutting off the supply of gas to the cylinder. Surplus gas and powder residues are evacuated through an exhaust port in the gas cylinder. This system ensures that only the precise volume of gas required to overcome the mass and resistance of the rifle’s moving assembly is admitted from the barrel.”

Theft is never acceptable.

Click on the glass to see how an AR15 DI works.
http://www.bushmaster.com/anatomy_bushmaster.asp

Piston rifle.
http://www.youtube.com/watch?v=WvX6sXs_ZO4

I find it silly that people keep saying that piston guns run cleaner then gas gun. They actually believe that the carbon magically disappear in a piston firearm. It doesn’t, the filth is deposit in a different place.

The folks at AAC tell me that the best lubricant to use with a dirty, suppressed firearm, such as anything full auto, is Synthetic, 2-cycle motor oil, but I haven’t tried it yet.