
Originally Posted by
constructor
I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
Exactly. On a DI AR as the gas enters the carrier key and the carrier the bolt receives forward pressure and the carrier rearward pressure as the carriers chamber fills with gas (at the same time since the bolt is the forward end of this chamber and the carrier is the rear of this chamber). When the chamber can no longer contain anymore the carrier moves rearward and this causes the bolt to unlock due to the track in the carrier (bolt connected to the carrier via cam pin) follows this track. By this time the chamber pressure has dropped so much that the empty case easily follows and stays attached to the bolt via the extractor. As the bolt reaches fully unlocked the gas rings pass by the two ports on the right side of the carrier and the gas is vented outside those ports.
One of the reasons for the H (3.8oz) and heavier CAR length buffers are to help delay the carrier from defeating the buffer and spring as soon as it would with a CAR (3.0oz) buffer, a heavier weight buffer is harder to defeat (simple physics). The CAR length gas guns are trying the move the bolt rearward faster than a longer rifle gas system does due to the shorter length and reduced gas system capacity. Because of the shorter gas system length the internal chamber pressure on the fired casing on a CAR gas gun is higher than it is on a mid length or rifle gas length gun, thus why the heavier buffers and stronger extractors help. Some of this can be controlled via the gas port in the barrel size. But it's still a timing issue, the system has to have enough gas to fill the gas system fast enough before the bullet leaves the barrel (dwell time). The stronger extraction devices (stronger extractor springs and O-rings) help keep the empty on the bolt due to this higher pressure on the empty case.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
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