
Originally Posted by
jmart
I never said or meant to imply the gasses immediately filled the tube/chamber in the BCG, I was simply stating that carrier movement, and subsequent unlocking, is initiated when gas is routed back to the BCG. Initial rearward movement occurs prior to the bullet exiting the muzzle. I suspect unlocking is completed prior to this event as well, but that's just a guess, but I'm making an educated guess that unlocking certainly is initiated prior to muzzle exit. Continued rearward movement is a product of the initial force to get it moving -- there's enough force and momentum to allow the BCG to continue its rearward movement, compressing the spring, and hitting the end of the extension before rebounding and returning to battery.
Agree that port diamter acts as a restrictor or valve to control the rate of gas entering the gas tube. The amount of dwell then controls the duration that the system gets charged. It's an engineering feat to get the right volume of gas back to the carrier to get it to cycle with the optimum velocity. Volume is a function of port diametr and dwell.
I don't see why it wouldn't. Once the system is charged sufficiently to overcome the resting state, the BCG will begin to move rearward. Bolt unlocking won't be initiated until the carrier moves rearward to the point the cam pin engages/rides the slot and begins unlocking as the BCG continues rearward. Once unlocked, momentum continues the cycle. But I'm not understanding why initial motion would be delayed until after the bullet exits the muzzle.
I can imagine that it takes some amount of time, but given the bullet's transit time down the bore of a carbine is about ~ 0.75 of a millisecond, as modeled above, I can only imagine that remaining time for gas exiting is a fraction of that. Gas velocity is quite high compared to bullet velocity. And as the gas exits, the residual pressure drops like a rock. Maybe not instantaneously when talking about ultra slow motion measuring devices, but much quicker than it takes for the BCG to extend fully rearward I would think.
Do you have any data which shows how a carbine cycling at 650-750 rpm, how much time it takes for the BCG to complete it's rearward movement? At 750 rpm, a shot occurs every 0.080 seconds if my math is correct. That's 80 milliseconds. Subtract 5-6 milliseconds for hammer fall and that leaves ~75 milliseconds inbetween shots. Now you have that entire time for the BCG to complete its back and forth cycle compared to the ~0.75 of a millisecond it takes for the bullet to travel the length of a carbine barrel. Assuming carrier travel consumes the largest portion of the time budget, the bullet is long gone, pressure has long ago fallen to zero, and remaining rearward movement is a function of initial force applied while the gas system was charged (that time between passing the port and exiting the muzzle) and momentum. That's the way I envision it anyway.
ETA: K.L. Davis' stickied thread "After You Pull the Trigger" states that unlocking occurs at the 375 microsecond point in a carbine. That's .375 of a millisecond. I'm not sure what event that's referenced from (primer ignition, bullet release from the neck or bullet passing the gas port), but if the latter, then unlocking would begin after the bullet exits based on the above graph. If one of the former, then unlocking is initated by the casehead pushing against the BCG, before the system is pressurized -- essentially it would be blow back induced at first, followed by a gas assist once the bullet passes the port. I still don't see what gas is available within the gas system after the bullet exits the muzzle, maybe it's due to inertia, and it takes several microseconds to get everything moving and for pressure to evacuate the muzzle.
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