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I'm looking into getting an AR, and I've read that the carbine length gas system isn't as reliable, etc... as the longer gas systems. How "true" is this? Is it something I should be concerned about or no big deal?
The argument says that a carbine gas system on a 16" or longer AR is "overgassed", which apparently has some theoretical damage imparted on bolt carriers and other internal parts over long periods of heavy use. If anything, the "overgassing" makes things more reliable as it will ensure the rifle cycles with that evil, underpowered steel-cased ammo from Russia.
There is another aspect of the discussion that says that midlength gas systems allow more real estate for rails and accessories.
There is LOTS written about this, both on this site and a simple Google search, and like many "coke vs pepsi" arguments, the discussions go in the same circle.
IMHO, the 'negative' aspects of the carbine system on a longer-than-14.5" barrel are so minor that 99% of AR shooters will never see the impact. If you want the extra space up front on the rails, then midlength is a good option.
Google often leads shooters to this site, which is about helping fellow shooters, which seems high road to me.
OP, the carbine gas system on the M4 is used on a shorter barrel too. Civilian models grew the barrel to 16" to be NFA compliant, but left the gas port in the same location. The mid-gas system has the added benefit of longer sight radius for irons (a diminishing factor due to the prolific use of optics). The result is lower chamber pressure when the bolt begins to unlock, thus the case is less sealed against the chamber walls and requires less force to yank out. Aside from the occasional broken lug, bolt failures in M4's are typically a cracked bolt at the cam pin location due to the forces exerted there during extraction.
There are other factors too including the ammo you are shooting, gas port size, bolt group/buffer weight, and buffer spring strength. Hell, there are even modified gas tubes that can be used to alter the dynamics.
Conclusion, I would go with the mid-gas, but don't be afraid of a carbine gas either. My middy shoots M855 and Tula equally well.
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