it would actually be a fab 10 but would it be accurate?

and how much do collapsable butt stocks cost?

  I've got an old CAR upper (11.5+5.5).  I've found it to be accurate enough for what it is.
It's the short sight radius that hurts you, not the fact that the barrel itself is short (that causes a lower velocity)

YMMV

If you used a scout type optic, then you wouldn't have to worry about the sight radius.  Though something that short isn't really meant to be used at 300 yards...

Collapsable stocks should be readily available in the $85 range.

My favorite rifle is an M4gery I built up on an Olympic lower, no-name upper, and various "gun show" parts of unknown quality.  But I used a 14.5" Colt barrel and Colt bolt/carrier.

The first time I took it out to the range, my buddy Russell came along with his factory Colt H-BAR.  Shooting Canadian C77 ammo standing off-hand through the iron sights at the 600 yard gong, I consistantly hit it about twice as often with the little M4 than my friend and his H-BAR.  (6 or 7 times out of 10, as opposed to 3 for the H-BAR.)  When we traded guns, he had no problems out shooting me every time, so it wasn't our marksmanship.

Needless to say, Russell couldn't believe that my little carbine shoots better than his H-BAR. He is now talking about converting his H-BAR into a carbine.

My advice is choose a good barrel and bolt (Colt or Bushmaster), and you'll have an accurate, reliable rifle.