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Posted: 4/12/2007 3:49:18 PM EDT
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I was looking at CMMG uppers. They list them with CAR, mid length and rifle gas systems. I understand the exterior physical differences between them but what are the functional differences between them? Is one preferable to the other? Also what is the difference between a "F" marked front sight and a standard front sight? thanks |
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F markings mean the FSB is mil-spec. A little info. Rifle gas system has a softer feel than mid or car gas system. The advantage is also that you get a longer sight radius. You can get a car system and just put a low profile gas block and rifle length handguards but the softer feel of the recoil from a rifle or even mid is nice. 3-gun shooters prefer the rifle length because they can get faster follow-up shots. Many of them use a 20" rifle though. For a 16" in 5.56 I think the mid gas system is figured to be ideal. Someone will come by with a chart that shows what gas system is ideal for whatever caliber and barrel length. |
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'F' front sight bases are for use with flat top receivers, as detachable carry handles have the rear sight located slightly higher than normal. There are two important dimensions in the AR-15 gas system: distance from chamber to gas port, and distance from gas port to muzzle. The first determines how high the gas pressure will be in the system, the second determines how long the pressure pulse will last. A short distance chamber to port means you will be closer to the peak pressure of the firearm, and also means the case in the chamber will have less time to shrink away from the walls of the chamber. Violent functioning, higher cyclic rates and extraction while under pressure are all increasingly possible as this dimension shrinks. A short distance port to muzzle means the pulse will be shorter, and will not have the same opportunity to accelerate the bolt group. Consequently, there is a chance the bolt will not get sufficient energy to completely cycle, and may short-stroke. In a simplistic way, you want both dimensions as long as possible. A very low pressure pulse, acting for a very long time, would gently accelerate the carrier and should give very good reliability and allow parts to last forever. A very high pressure pulse, with a short port to muzzle distance, would accelerate the bolt very quickly for a short period, and may still give only marginal cycling while kicking the snot out of the parts. A carbine with a 14.5" barrel, mid-length with 16" and rifle-length with 20" all have the same port to muzzle distance, but varying chamber to port length. |
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