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Posted: 12/15/2012 6:38:56 AM EDT
| I am gathering parts for my first build and trying to decide on a barrel. I'm looking at a Rainier Arms 18" polygon with a mid length gas system and a Black Hole Armory 18" polygon with a rifle length gas system. I will be using this mainly for target shooting and maybe start getting into some match shooting. What would be the difference between the two? |
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Quoted:
I am gathering parts for my first build and trying to decide on a barrel. I'm looking at a Rainier Arms 18" polygon with a mid length gas system and a Black Hole Armory 18" polygon with a rifle length gas system. I will be using this mainly for target shooting and maybe start getting into some match shooting. What would be the difference between the two? The most obvious difference is the gas system length. The relationship of the gas port in the barrel to the muzzle end determines the gas pressure that is returned to the bolt carrier via the gas tube. The carrier needs a certain amount of energy (gas pressure) to cycle the rifle. The closer the gas port is to the muzzle end of the barrel, the less pressure there will be to cycle the rifle. As the bullet travels down the barrel and passes the gas port, the gas port is now exposed to high pressure gasses which supplies the gas system with the needed pressure to cycle the bolt carrier. When the bullet exits the barrel, the gas pressure in the barrel drops to nothing. So, the length of time that the bullet is still in the barrel after it passes the gas port dictates how much pressure is allowed to get into the gas system. - This length of time is known as "dwell time". A 20" barrel was designed with a "rifle length" gas system which places the gas port the proper distance from the muzzle for the proper amount of gas pressure to cycle the rifle. If you have an 18" barrel using a rifle length gas system, the pressures returned to the bolt to cycle the rifle will be less because the gas port will actually be 2" closer to the muzzle end. Lower pressure will result in less felt recoil, slower cycling rifle, possible short stroking which could cause feed problems, etc.... One way to try to compensate for low pressure is to use a lighter buffer, another way is to enlarge the gas port on the barrel to allow more gas into the gas system. In my opinion I wouldn't use a rifle length gas system on an 18" barrel. That's just my opinion tho, they wouldn't make an 18" rifle gas system barrel if it didn't work. I just think that it wouldn't be 100% reliable. I don't know the actual specs, but a mid length gas system on an 18" barrel would produce comparable gas system pressures to a rifle length gas system on a 20" barrel. An 18" barrel with a mid length gas system would have more of the gas pressure needed to operate the rifle and in my opinion would be more reliable. That's my story and I'm sticking to it. ** DISCLAIMER ** I ain't no expert. |
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