This will be my first build, so I know almost nothing. I'm taking lots of notes in this thread. I just know I want 18.6" in 223 and for the entire rifle to be relatively light (8ish lbs)
Question: What is the difference between mid-length and rifle-length gas system? I have googled a bit, and got mixed results. Which would be preferable for an 18.6" barrel?
So the difference is the length of the gas tube (obviously), which has important results when it comes to how the rifle functions. A rifle-length gas system is usually done in barrels 18 inches and longer (but some use mid-length up to 18).
The main difference between the two is known as dwell time. This refers to the amount of time that there is pressure in the barrel that allows gas to keep going back into the bolt carrier group to have it cycle backwards.
This difference has two main results - one, it affects how much recoil the shooter feels, and two, it affects reliability when it comes to the type of ammo the rifle can cycle consistently.
For example, rifle-length gas systems in barrels 18 and above are known to be very soft shooting but are often finicky with the ammo they will reliabiy cycle, because there is less time available to cycle the bolt carrier when compared to a 18" barrel that has a mid-length gas.
As a result, softer powered ammunition (less powder and less pressure) may have a problem being reliable in a rifle gas system because there may not been enough gas pressure to fully cycle the bolt backwards to the rear and to properly take another round out of the mag and into the chamber.
This is known as short stroking and I have personally had this problem when shooting Independence brand 5.56 in my 18" rifle gas AR. It short stroked almost every round on an H2 buffer. In comparison, black box American Eagle XM ammo worked just fine because I assume it's loaded a bit hotter.
A lot of people say that mid-length gas systems are the best compromise between reliability and recoil feel when it comes to 5.56 / .223 ammunition.
It's a whole can of worms if you also want to talk about the effects of how buffer weight (milspec rifle buffers vs carbine buffers vs rifle buffer springs vs carbine buffer springs vs H2, H3 buffers). All of these things have an effect on reliability and recoil as well.
Even different calibers run better with different gas systems. For example, more barrels chambered in 300 Blackout have pistol-length gas systems, while 7.62x39 have carbine-length gas because they work the best with those calibers.
I know it's as clear as mud but I hope it helps.
For what it's worth, I would go with a mid-length gas system and start off with a milspec buffer (either rifle or carbine-length depending on if you're using an A2 length stock or an adjustable stock) and then play with it from there if you want to optimize your setup.
