Originally Posted by deg
So, I assume I have a carbine?
Sure looks like it. Remember that the gas tube actually starts behind the bolt face. Take a look inside your mag well or ejection port and you will see it sticking out where it interfaces with the tube on the BCG gas key.
The fwd end of the gas tube also enters the forward sight block about 1/4 inch. this gives you a much more accurate measurement. . but it looks like you have it.
You also have the delta ring and fwd end cap that retains your foregrip or handguard. If you want to go to a fancy full-length handguard, then you will need to deal with removing the FSB and EndCap..
Otherwise, you can pick-up a railed carbine length in the style that snaps in. Your barrel won't be free floated but will still look cool, kool, kewl....
You can never really achieve a free float with a gas tube anyway, but we all like to think it is.