I think a discussion of AR-15 gas system lengths deserves it's own post and this is it. The big issue here is barrel length and port pressure needed to cycle the gun. As the gas port move closer to the gun's chamber (and away from the muzzle), the port pressure goes up and the gun cycles harder. This is good in that it is more likely to cycle, but bad in that too hard will break things.
Rifle length systems are about 12 inches long. As they are the longest, they are also typically the gentlest of the gas systems. But if the barrel is too short, then they won't cycle properly because the gas port pressure will drop too low after the bullet leaves the barrel. Because of this they are typically used on guns with barrels 20 inches or longer like the M16.
Carbine length systems are about 7 inches long. There are shorter lengths (like the pistols), but for most people this is as short they want to go. These cycle the action quite hard. Hard enough that Colt had to make changes from the M4 to the M16 to beef up some parts, alter the feed ramps, and use a heavier buffer to slow the bolt down. These are typically used on guns in the 14.5 to 16 inch range like the M4 carbine. However civilians can't have barrel lengths below 16 inches without a special tax stamp, so most people with carbine length gas systems have 16" barrels or 14.7" inch barrels with longer pinned flash hiders to meet the 16" legal requirement.
Midlength systems are in the middle and are about 9 inches long. These shoot softer than carbine, but harder than rifle. It's a happy medium and works very well for 16 inch guns. Since I have a 16 inch gun, this is what I bought. The disadvantage is that this is a purely civilian length gas system and has less support than the rifle and carbine lengths in the aftermarket. Until recently only one company made a free float tube for it. About the only accessory my midlength can use that a carbine can't is a standard military bayonet.
And for the record, I think the 14 inch barrels that are pinned to 16 inches are silly. The 5.56 round need all the barrel it can get to tumble and fragment for proper lethality. I also think the stepped barrels on 16" guns are silly. Military carbines have that feature so you can mount M203 grenade launchers on them. Civilians can't own those either.