And than added a FS for a total length of about 17" or so .
I think this would be a nice set up given that it would keep the full size handgaurds and sight radius, but still be compact.

Or would that hose the gas system up?

KKF does it, I assume he does something with the gas system to make it work right.

Uh, a Dissipator?

Quoted: Uh, a Dissipator? www.bushmaster.com/shopping/weapons/Images/pcwa2s16d.jpg

Close , but it doesn't use a full length gas system if I remember correctly.

Correct me if the information below is wrong, but this is how the AR gas system seems to break down in function.

With the gas port at the end of the barrel, you have the problem that the bullet will leave the barrel and release the pressure before the gas tube pressurizes fully, cycling the bolt carrier. Basically, the bullet needs to be in the barrel for a certain amount of time, after the gas port for the carrier to recieve the correct amount of gas from the gas tube, to cycle. If the time is too great the carrier will cycle early, and if its too short it reduces the reliability and could cause short stroking.

This is why a dissapator uses a carbine length gas system. And why the 20-24" use a longer system.

So it is Dissy or no go than ?

Check with the boys at CMMG, I got a Colt dissy upper that they had made. It runs like a raped ape