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Posted: 12/12/2013 7:39:26 AM EDT
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I am working on my first build and need help deciding on which barrel.
So far I have the following -Aero precision upper and lower -stag lpk and reciever extension -dd bcg -dd ris ii quad rail -magpul ctr I can't decide between dd 16 mid length gov profile barrel or a dd 14.5 pinned welded carbine barrel. Range / home defense use What barrel would you guys choose and why? |
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For either barrel, look for a 14.5" carbine gas system barrel with a 0.063" gas port, or a 16" mid-length gas system with a 0.071" gas port. Also, once you obtain a gas tube verify that the inside diameter of the gas tube is 0.120" (this is the diameter the mass produced AR15 used; circa 1958-1960). These port diameters are based off the 14.5" / 0.063" carbine that has a port pressure of 26,000 psi, and a 20" / 0.093" rifle that has a port pressure of 13,000 psi.
With this information we can conclude a 16" mid-length's optimum port diameter would be 0.071" when using ammo that has a chamber pressure of 55,000 psi. This is because they all share a dwell time that uses a length of 7" from port to muzzle. A 14.5" carbine has a port that is 7.5" from the bolt face, a 16" mid-length has a port that is 9" from the bolt face, and a 20" rifle that has a port that is 13" from the bolt face. The 14.5" carbine' port is 0.030" smaller than the 20" rifle's port. Using all of this information, we can see that a difference of 5.5" from the bolt face to port exists between a 14.5" carbine' and a 20" rifle. If we divide 0.030 by 5.5 it equals 0.00545455. Now, since a difference of 1.5" exists between a 14.5" carbine and a 16" mid-length between bolt face and port; 0.00545455 times 1.5 equals 0.00818182. Therefore, adding 0.00818182" to 0.063" equals 0.07118182". As for my setup, I run a 14.5" carbine gas system with a 0.063" (port diameter Colt specifies for USGI M4). The upgrades I make use of are a 5-coil extractor spring w/ black insert & viton o-ring, an enhanced action/buffer spring, enhanced ejector spring (all 3 springs are from Tactical Springs [subsidiary of Sprinco USA], and a H2 buffer. I based my decision on most of these upgrades due to Mike Pannone's article The Big M4 Myth. I do plan on getting a H buffer to convert my H2 into a H3. The with the remaining parts I will put together a carbine buffer for backup. |
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