Well??? I was just thinking... Has anyone done it or is there a mock-up of what it would look like? I'm sure it would look weird.

I'm no engin-nerd or a gunsmith by any means, but common sense tells me that the length of the gas system has a relationship with the movement of the BCG, much like the relationship between ignition timing and valve timing in an engine. If this timing is disturbed by changes like what your saying I can imagine that you run the risk of having the bullet still in the barrel when the BCG starts to unlock. Again, not an engineer here but I always figured thats how the system works. I mean there isnt that much difference between a carbine 14.5, midlength 16 and rifle 20 in terms of distance from the gas port to the end of barrel. But I would think that a carbine gas system on a rifle length barrel would throw that off by a substantial percentage.

I think its a bad idea.

Overgassed and a more violent recoil impulse.  There is a reason people have 14.5 midlength and 18" rifle and not 20" carbine gas.

Depends on how big your gas port is.

Sabre Defense used to sell some barrels like that.  They had more length from the gas port to the muzzle than from the gas port to the chamber.  It really made me question weather those guys had any idea what they were doing.  

What happens is that the timing is much too early causing the bolt to unlock while the pressure in the chamber is still very high.  It's very hard on extractors, cam pins, etc., and offers no benefit.

Interestingly DPMS is selling 16" rifles and barrels with rifle length gas systems which makes much more sense, especially if you plan to use a suppressor.

I understood there was a reason just not exactly why. That is why I asked.

It won't necessarily be "overgassed".  The gas port could be small enough to restrict the gas volume to a reasonable level.  It's really a timing issue.

Not really. You can tweak this some, but the pressure curve of ammo cannot be completely compensated for if its out of time just by "opening it up." If it didnt matter, there would only be one length of gas system on an ar regardless of barrel length.

Anyone have pics I've looked but I couldn't seem to find them. I just think this would look really wonky.

what would happen is it would be called a dissipator Bushmaster way back did barrels like that carbine gas system on a 16" barrel that had the FSB closer to the muzzle for a carbine legnth but longer sight radius.

Carbine gas on a 16" barrel is fairly normal....

im thinking carbine legnth on a carbine barrel with the FSB way forward for longer sight radius...forget what I last said

OP needs to read more.

The knowledge you seek is out there and easy to find without resorting to "Hay Guys...."  spend time in the troubleshooting forum and the old stickied threads.

The same problem that guns designed to run with suppressors have when they aren't mounted, or how some companies have oversized gas ports in the .090 range. Especially with lighter buffers. Massive overgassing.
If bad enough, it won't strip the next round from the mag or lock back on an empty because it's causing the bolt group to move to fast.
If it runs, eventually the trigger and hammer pins can break and the holes in the receiver will get out of round... this is the more common reason lowers get out of round trigger and hammer pin holes. Not from the pins rotating, but because the bolt is coming back so hard that it's slamming the fire control group much harder than it should be and that force is being transmitted to the receiver. Another reason I don't like the anti-roll pins... they can actually mask a bigger problem, and will do nothing to help this specific situation.
Parts will wear out much faster. The bolt will break sooner... it's already the reason that carbine bolts don't last as long as rifle bolts. They have a higher incidence of breaking lugs, or at the cam pin hole than the ones in rifles.

Or ...you could just ignore the thread and not click on it if it bothers you so much...or better yet not even waist your time commenting

Thanks, Your wonderfully informative post helped me greatly.... Also, Your link that you posted was extremely helpful in showing me that a little research would solve all of my issues. God, I did do some research looking for pics of something like this. Why? Because I 'm curious how it would look. I'm sorry that since my search didn't turn up anything and god forbid I come here asking for other people to tell me some stuff because I didn't find it.

Here is the information I offered in a post a week prior that also was interested in what would happen using a carbine port with a 20" barrel. I would start by reading the 3 papers below.

COMPARISON OF A THEORETICAL AND EXPERIMENTAL STUDY OF THE GAS SYSTEM IN THE M16A1 RIFLE
TECHNICAL NOTE 48: THE EFFECTS OF BARREL DESIGN AND HEAT ON RELIABILITY
THE GAS- FLOW IN GAS-OPERATED WEAPONS

Thanks I really like the second one the other two were very sciency (yes it's a word ) but reading the summaries and how they went through the process I think I get it.

You either cannot comprehend what he's asking, or you didn't read. either way, this is not anywhere close to what OP is talking about