My understanding is that the more barrel length you have beyond the gas port the more gas gets fed into the system, making the rifle cycle more forcefully.  So I would think a 10.5" or 11.5" SBR with carbine length gas would be undergassed compared to a 16" or 14.5 with the same gas system, but I see where all kinds of people are putting heavy buffers in their SBR's.  I would think a regular Carbine buffer would be sufficient, but I must be missing something.  If anything wouldn't you want a lighter buffer than a regular carbine?

The gas port diameter is usually drilled larger than required to allow the use of all sorts of ammo with it. If you shoot exclusively hot ammo and your sbr has a large gas port, you'll get too much gas, which can cause wear and tear. You can use a heavy buffer to slow it down, but you'll have more felt recoil.

Alternatively, if you use an adjustable gas system and a light buffer, you'll have less wear and tear and less recoil from the lighter moving parts.

You've got it backwards:  the shorter the barrel the closer the gas port is to the chamber and the sooner the bolt functions and at a higher pressure.

Sometimes, to get things calmed down a heavier buffer is required.

A while back we had some CQBRs (10.3") and they were a bit finicky...our armorer tweaked some of the problem children with heavier buffers.

double tapped.

Barrel length is irrelevant to the location of the gas port relative to the chamber.  Gas system length is where the gas port is relative to the chamber, and the OP specifically mentioned different barrel lengths which use the SAME gas system length.

10.3/10.5" barrels are notoriously flaky.  High gas port pressure, late timing, and minimal dwell time mean that minor variations in gas port size, etc can noticably impact weapon function.  It's common to have to tweak these by trying different buffer weights.  Realize that by default most manufacturers overgas everything, this is why heavy buffers are a commonly used and needed item.

Buffer weight is just one variable in a list of variable factors that all go into and impact in different ways the operation of an AR type firearm.  If someone has the idea that "heavy buffers are always better" or "are better for SBRs" or any other generalization, they are incorrect.

http://www.ar15.com/forums/topic.html?b=3&f=12&t=514521
Just yesterday for example, here is someone shooting a 10.5" barrel AR and is running a heavy buffer which is almost certainly creating malfunctions.

If your brass is ejecting forward, you need a heavier buffer/bolt carrier or less gas.

If you brass is ejecting backward, you need a light buffer/bolt carrier or more gas.

If your brass is ejecting between about 2 and 4 o'clock, you don't need to mess with it.

Yet another issue with someone using an ST-2.  These buffers are causing alot of problems.  There is a thread a few posts up where someone has an issue with an SBR and an ST-2.  Weight alone will not help.  You have to look at how they work too.  A standard buffer has 3 weights that move.  The movement is what does the buffering.  An H buffer has 3 weights with one of them being tungsten (heaver).  The H2. 2 of the 3 are tungsten.  H3.. has 3 tungsten.  The ST-2 has powder or shot.  I'm guessing it doesn't move right. Also causes bad carrier bounce.  A 9mm buffer is the heaviest, but no buffer weights moving.  A 9mm AR is blow back operated not gas like 5.56.

Jk

Ok, so as a GENERAL statement most carbine length gas systems tend to be overgassed at 16" or 14.5, and many have the gas port drilled so large that they're still overgassed at 11.5 or 10.5?  So, in building my SBR (an 11.5) I'm thinking start with a standard carbine buffer and go from there.  It should be pretty much guaranteed to run with a carbine buffer, but if it seems overgassed I can always go heavier, right?

My 11.5" has a .071 gas port and I run a 9mm buffer and FA carrier with it.