Recently read a magazine describing direct impingement gas operation in the AR. It was an article on a piston gun and seemed to have left out some information. I've noticed a lot of confusion and out right misinformation on the subject, so let's clarify what really happens.
Does the DI system direct gas into the action? Yes - there's no denying it. How much seems to be the issue. In the article, and in public perception, hot, nasty, dirty gases are sprayed all over the bolt, upper, and everything in between. It's not quite that bad, folks.
Nothing happens until the bullet passes the gas port in the barrel, right? Then the opening is uncovered to allow gas to pass through a .063 hole into the tube, and from there to the gas key in the bolt carrier. At this point (the article fails to mention,) the gas is directed into the chamber behind the bolt, which is sealed by the gas rings. The pressure pushes the bolt carrier back, rotating the bolt, which is unlocked.
Meanwhile, the bullet hasn't stopped moving. The whole point of the gas port location and size is to time the bolt opening until AFTER the bullet has left the barrel. Otherwise, the case would be subjected to high pressure and prevent extraction.
It doesn't have high pressure because the barrel is unobstructed by the bullet and has a gaping .223 hole in it. Which will pass pressure and residuals more, a .063 gas port, or a .223 muzzle?
THEN the bolt carrier moves backward, and before the gas key and tube separate, the chamber is partially cleared, allowing residual gas pressure to escape past the brass. Nearly ALL semi auto's do that, as the cartridge case is pushing the bolt carrier back to power the action. ALL blowback actions do that - it's the only way they work.
So the gas entering the chamber and action comes largely past the case from the .223 throat end of the barrel.
NONE of the gas escaping out the muzzle is directed into the action, it's open first, and substantially bleeds off pressure before the action is even beginning to extract the case. The bolt carrier has to move at least an inch to uncover the gas tube end, pressure is substantially reduced at that point, and both ends of the barrel are venting the majority of what's left. Two big holes pass more volume and pressure than one tiny hole.
The hot, nasty, dirty gases are not being totally vented into the action by the gas tube. It's a small fraction, and a lot comes from around the case during extraction. As for the pressure IN the bolt carrier, it's vented out the side holes facing the ejection port. After the first shot, there's no port cover to constrain it - and there isn't much to hold back. I've yet to see much burnt powder on one. The pressure is so low it won't open the cover, the bolt carrier does.
How much pressure is needed to cycle the action? Can your children charge the bolt on an AR? Sure. It's obvious it doesn't need much, and not that much gas is tapped to simply unlock the bolt. The spring and buffer arrest most of it, leaving about 25 pounds of recoil out of the 50,000 pounds generated.
It's the same with piston guns. The brass on those comes out dirty, too, and the same on roller locked actions. Blow back pistols and .22's, dirty. Gas passes around the case and gets into the action regardless. Only a bolt, pump, or lever won't.
The DI system isn't dumping all it's gas into the action, and those who perpetuate that misconception are either uninformed or selling something. Nothing wrong with buying and owning piston guns, most semi autos are. Selling the idea they are substantially cleaner, not so much. Like most things, it's simply a matter of degree.
It's just a .063 hole's worth.
Does the DI system direct gas into the action? Yes - there's no denying it. How much seems to be the issue. In the article, and in public perception, hot, nasty, dirty gases are sprayed all over the bolt, upper, and everything in between. It's not quite that bad, folks.
Nothing happens until the bullet passes the gas port in the barrel, right? Then the opening is uncovered to allow gas to pass through a .063 hole into the tube, and from there to the gas key in the bolt carrier. At this point (the article fails to mention,) the gas is directed into the chamber behind the bolt, which is sealed by the gas rings. The pressure pushes the bolt carrier back, rotating the bolt, which is unlocked.
Meanwhile, the bullet hasn't stopped moving. The whole point of the gas port location and size is to time the bolt opening until AFTER the bullet has left the barrel. Otherwise, the case would be subjected to high pressure and prevent extraction.
It doesn't have high pressure because the barrel is unobstructed by the bullet and has a gaping .223 hole in it. Which will pass pressure and residuals more, a .063 gas port, or a .223 muzzle?
THEN the bolt carrier moves backward, and before the gas key and tube separate, the chamber is partially cleared, allowing residual gas pressure to escape past the brass. Nearly ALL semi auto's do that, as the cartridge case is pushing the bolt carrier back to power the action. ALL blowback actions do that - it's the only way they work.
So the gas entering the chamber and action comes largely past the case from the .223 throat end of the barrel.
NONE of the gas escaping out the muzzle is directed into the action, it's open first, and substantially bleeds off pressure before the action is even beginning to extract the case. The bolt carrier has to move at least an inch to uncover the gas tube end, pressure is substantially reduced at that point, and both ends of the barrel are venting the majority of what's left. Two big holes pass more volume and pressure than one tiny hole.
The hot, nasty, dirty gases are not being totally vented into the action by the gas tube. It's a small fraction, and a lot comes from around the case during extraction. As for the pressure IN the bolt carrier, it's vented out the side holes facing the ejection port. After the first shot, there's no port cover to constrain it - and there isn't much to hold back. I've yet to see much burnt powder on one. The pressure is so low it won't open the cover, the bolt carrier does.
How much pressure is needed to cycle the action? Can your children charge the bolt on an AR? Sure. It's obvious it doesn't need much, and not that much gas is tapped to simply unlock the bolt. The spring and buffer arrest most of it, leaving about 25 pounds of recoil out of the 50,000 pounds generated.
It's the same with piston guns. The brass on those comes out dirty, too, and the same on roller locked actions. Blow back pistols and .22's, dirty. Gas passes around the case and gets into the action regardless. Only a bolt, pump, or lever won't.
The DI system isn't dumping all it's gas into the action, and those who perpetuate that misconception are either uninformed or selling something. Nothing wrong with buying and owning piston guns, most semi autos are. Selling the idea they are substantially cleaner, not so much. Like most things, it's simply a matter of degree.
It's just a .063 hole's worth.
