Outlaw recovery,
The AR was originally created with the 20" barrel and to work the best in that configuration.
Now with all the variants every day including, pistols, car uppers, pistons, supressors and what not all that you need is a well tuned system. It is ok to have a carbine if that is what you like but there is a risk if you could end up overgassed. There is another interesting variant to the 16" that is the disipator upper that it is basically a barrel ported like the same 20" length with the longest sight radius but in 16" and still some of the benefits of the classic 20" system in 16" length.
If your brass ejection pattern is 1 to 3 o'clock there is a good chance you are overgassing. Look also for buffer and spring options but the most important part is to get the extra gas down.
If you want a CAR system I would seriously consider these two options in any build:
A) A adjustable gas block. This allows you adjust your dwell time.
B) Adams piston system. (This comes with 3 settings, normal, suppresed and cancel)
So with option A you can start with the closed port and the mildest ammo, (for example a PMC bronze load) and then open the screw 1/4 turn at the time until the carbine starts cycling properly. The proper ejection pattern shoudl be about 3 to 4:30 o'clock. The other way is to take your average and NATO loads and do the same but then your system will not cycle with the mildest training loads. There are gas blocks that come with a several position key for the impigment system, just exactly as some pistons do but I am not familiar with those.
One other advantages of the mid and disipator type is that the further away you are from teh chamber the gases cool off a bit and then when they come back into the bolt they come cooler. Might not be significant in normal use but in a suppressed mode you can tell by the time it takes for the whole group to come up to grilling temperature. That is when the open bolt configuration (machine gun) should be called in.
The piston also has many benefits in any configuration but it is not always necessary as some claim. I have all variants and there are things that I like and also dislike of each one.
Any system needs tuning therefore idealy any system should provide the capabiltiy to be tuned up to get the best out of it.
Cheers,
E.