When building an AR your barrel length is one of the main choices. When you choose your length, you have to make sure that you also choose the right gas block tube length too. The gas blocks come in many different lengths.
The shorter your gas system, the higher the pressure it's has. The bolt and carrier are slammed harder; they work harder. Plus, a shorter gas system is harsher in operation, because it cycles in a shorter time.
Rifle length is the original system that the gas system is designed around. With a rifle length, an aluminum gas block is fine, which makes the rifle lighter. With a carbine length, the pressures almost double, and an aluminum gas block will eventually wear down by the hot gas. You should use a steel gas block system on carbine length or mid-length rifles.
As the gas systems get shorter, the size of the gas port needs to get bigger, because they need to bleed off more pressure in a short time. If the gas port is too small, the rifle will short-stroke, causing feeding problems.
On the other hand, if the port is too big, the rifle will be "over gassed." Which will send the gas to interact with the bolt too early,with the shell still in the chamber and will cause it NOT to eject.
An over gassed rifle will be really difficult and uncomfortable to shoot. This cart will help you get a better idea of the relationship between the distance of the gas block and the pressure.
Look at the chart below:
Gas System Gas Port Location Pressure (PSI)
Pistol 4.7" 48,000
Carbine 7.8" 33,000
Mid-Lenght 9.8" 26,000
Rifle 13.2" 19,600
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