Quoted:
Funny, my Postman just dropped off that same GB a few minutes ago.  It is slightly less than 1" long.  I got it for a carbine using a YHM midlength diamond FF rail.  The YHM endcap wouldn't fit with the longer gas blocks, so I went with that because it is so short.

To answer your question:  No a carbine length won't cover it.  It will stop right at the gas block shelf on your barrel.  You need at least a 9" (midlength rail).  10" or spector will also work.  If A2 FF refers to the rifle length, measure out about 11-12" from your receiver to see where it end.

Look at the pictures in the third post down on this thread:

http://www.ar15.com/forums/topic.html?b=3&f=4&t=446549

Quoted:
Yes, it fits.  Same one I used for these 3 builds with YHM lightweight FF tubes:

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It is VERY easy to install.  Make sure you use a feeler gauge to space it 1/32" away from the barrel shoulder, as if the handguard end cap were still inplace.  I know a guy who took an old handguard end cap, cut in half across the middle, and uses the two pieces as his depth gauge....works every time!

To index it, I use a mechanical pencil and a steel ruler; I draw a pencil line across the gas tube hole on the barrel, so it sticks out across where the gas block is to be installed (making sure the line is indexed across the top of the barrel, of course).  Then, I draw the same lines across the face of the gas block so all I have to do is line up the pencil lines (or "tic" marks).  Works every time!

Something else I like to do is to temporarily mount the gas block in place with the set screws, and then remove it...hopefully, when you tightened the set screws they left small marks where they will mate up with the barrel surface....then, I use a 1/8" drill bit on slow speed to make small countersunk depressions for the set screws to set in.  This will keep the GB from rotating if you ever need to remove the gas tube roll pin, in the event that the gas tube roll pin is a little tight (ask me how I came out having to do this....oops!).  It works, just don't drill very far, LESS IS MORE.  All you need to do is provide just a slight indentation to give the set screws a nice anti-rotation mounting surface.

Hope this helps, good luck!