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Post by slosqueezin on Dec 12, 2012 19:31:26 GMT -8
I hear about gas system lengths and need to better understand the concept. While there may be options, what is the least troublesome gas system length for a 20" or longer barrel ? If I pick the "long" gas system is there a down side ? If there is a down side, how do you fix / deal with it ? Any clarification would be appreciated, thanks.
Hank
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dr69er
Senior Member
Protect the US Consitution & the American Way.
Posts: 222
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Post by dr69er on Dec 12, 2012 21:37:46 GMT -8
Under normal circumstaces the following usually applies:
14.5" to 16.5" Bbl. length, go with a carbine length gas system.
17" to 19.5" Bbl. length, use a mid-length gas length system.
20" and longer Bbl. nomally use rifle length gas system, but you can also use a mid-length system especially if your case capacity is minimal for your Intended loads.
So, the combination of gas volume, port pressure, bore diameter, powder burn rates, and dwell time all factor in the equation. Also, you can use different buffer/buffer spring weights and rates to adjust your timing/cycling of your AR-15. Sometimes, a good quality adjustable gas block makes sense to fine tune your gas system to your loads.
Good Luck, YMMV.
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Post by slosqueezin on Dec 13, 2012 6:09:44 GMT -8
Thanks for the explanation. I've heard people using either adjustable gas blocks or heavier buffers to control the cycle speed but only when using 16" barrels. I'm looking at the several 6mm options and a 20" barrel so it appears I may not need either if building a 6x45 or 6x68.
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Deleted
Deleted Member
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Post by Deleted on Dec 13, 2012 11:39:28 GMT -8
If a barrel is properly ported then you do not need anything else unless for special purpose loads and situations. The first thing is also to make sure you get a clamp on gas block that provides a perfect 100% seal and even pressure all around. When you get a 16" barrel always avoid the carbine length porting option whenever possible. That can cause more headaches than benefits and was one of the several problems of the M4 design that should had never made it into both the military nor the civilian fashions. Another issue, not related to the port but related to "why" popularly established non sense designs are the threads. The 1/2 inch threads are not helping, specially if you have a 6mm bore make sure you up the threads to 5/8x24 standard.
These things are not 100% necessary but if one's goal is reliability and to maximize in accuracy potential those a couple of no brainers to consider.
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Post by eisenochs on Jan 6, 2013 19:08:13 GMT -8
When you get a 16" barrel always avoid the carbine length porting option whenever possible. That can cause more headaches than benefits and was one of the several problems of the M4 design that should had never made it into both the military nor the civilian fashions. ... These things are not 100% necessary but if one's goal is reliability and to maximize in accuracy potential those a couple of no brainers to consider. I'd like to add one quick little caveat here-- carbine-length systems are great if you're doing a gas piston build. Although most manufacturers have successfully engineered the problem of carrier-tilt out of their systems (please note that I said most), this is an issue that seems to happen more with mid-length and rifle-length systems than carbine-length systems. In general, it seems to be a good idea to go short for pistons, long for DI, hence why I'm insisting on a rifle-length gas system for my current DI DMR build.
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Post by m4weave on Jan 7, 2013 4:45:27 GMT -8
Under normal circumstaces the following usually applies: 14.5" to 16.5" Bbl. length, go with a carbine length gas system. 17" to 19.5" Bbl. length, use a mid-length gas length system. 20" and longer Bbl. nomally use rifle length gas system, but you can also use a mid-length system especially if your case capacity is minimal for your Intended loads. To go along with the advice above, the chart here on dwell time ~3/4 of the way down the page is worth a read, IMHO: ar15barrels.com/prod/operation.shtml(Not that I am endorsing another manufacturer, but it is good technical info that has been referenced here before I think.) Good luck! -Mike
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