The closer to the chamber a gas port is located, the higher the pressure of the gas used to operate the system. Why then, does a carbine length gas system require a larger diameter gas port compared to a rifle length?
The closer to the chamber a gas port is located, the higher the pressure of the gas used to operate the system. Why then, does a carbine length gas system require a larger diameter gas port compared to a rifle length?
Last edited by MistWolf; 10-31-10 at 18:29.
Do you need to match the front sight base/gas block port size to the barrel ports on sub 14.5 inch set ups or is it only the port in the barrel that varies?
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its based on the gas/dwell ratio.. the less dwell you have, the larger the port needs to be to compensate. to compare two different carbine systems:
a 16.1 with a carbine barrel needs to be a maximum of .062, because it's got a huge dwell bias of .465
a 10.5 needs a larger gas port, closer to .073ish, because it's got a huge gas bias of .714
you'll also note, however, that the largest gas port of all happens to be on the longest system- 20" rifle, with ports in the .090s, depending on who's making it. this is because bore pressure drops enough during the long travel down a 20" barrel that that's just what it needs to reliably cycle. if 20" barrels had carbine-length gas systems, the port size would need to be totaly different.
so, as stated, it's not the gas system, it's the ratio.
Last edited by bkb0000; 10-31-10 at 19:45.
Ok. In general- Larger gas ports for systems with shorter dwell times.
Larger gas ports for longer systems because pressure is less.
Would it be safe to say that posts I've read in the past stating larger ports are needed because the gas system has been shortened are either A) not accurate or B) I mis-understood what was written?
14.5" barrels have the same amount of barrel past the gas port as 20" rifle gas barrels...by design.
Shorter barrels than 14.5" will require a larger port to allow the gas system to fill with gas before the bullet leaves the barrel because everything is going to happen faster than would be needed if there was more barrel beyond the gas port. This is why 11.5" barrels are generally more reliable than say 10.5" barrels. The gas system has more time to fill and begin operation before than bullet leaves the barrel vs. the same size gas system on a shorter barrel with less barrel after the port.
14.5" barrels call for a gas port of .062". Rifle gas systems I've seen vary from .090-.100". These are larger because the gas tube being longer causes the entire gas system to be of larger capacity and the gas pressure is lower since the port is further from the chamber....simple physics.
All FSBs and gas blocks should have the same size port. Gas volume and pressure is controlled by the length of the gas tube and the size of the gas port in the barrel.
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people over generalize, too.
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So...I've got a mid-length 14.5". From what I understand, I would have a larger gas port than a 14.5" carbine length?
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Correct. There is less time for the bullet to act as a plug before it leaves the barrel. A larger port allows for more volume/second and reduced pressure right at the gas port. Because gasses are compressible, they will never flow faster than mach 1 through a nozzle. It would be interesting to do a dynamic analysis of the pressures at different points because again, since gasses compress, the peak pressures will be different at different points. The rifle still requires a minimum pressure to operate. Since a midlength gas system has a greater volume it needs more volume of gas to meet that minimum (Boyle's Law)
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