Quoted: I saw the mid-length pattern develop while reading all the threads in this section. Knew there had to be good reason for it.
As far as the increased recoil impulse, does this mean more recoil becomes sharper and the damage stemming from rattling the gun shot per shot?
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The AR15 and the components it uses (bolt, buffer, etc.) were designed around the gas pressures found in rifle length systems. When you go to a carbine length gas system, the gas pressure at the gas port goes from around 15,000psi (rifle) to 23,000psi (new carbine) and can easily hit 30,000psi (carbine w/ 9k rounds) with the increased gas port erosion. This means that not only is your bolt receiving a lot more gas pressure during cycling, it is also trying to cycle earlier than a rifle while pressures in the barrel are still higher and the brass is still gripping the camber walls. All of this is hard on the bolt, extractor and ejector and shortens the life of these components. In addition, the carbine cycles faster - meaning that your magazines have less time to properly position the round before the bolt carrier scoops it up.
The midlength gives you the same barrel length as a carbine; but reduces the gas pressure to more rifle-like levels by moving the port forward. This should result in a little better reliability and durability. You also get more sight radius, more rail real estate, and a smoother recoil during follow-up shots. Even better the worst compromise you have to make to get the extra performance is only about 1oz worth of extra handguard weight.