I am in the process of building a shorty AR, and noticed every little bell and whistle is adding length to my "shortie project".

7.5 inch barrel and a receiver of course...
Carbine buffer tube and a 2 inch flash supp make a total length of like 23 inches.

Maybe I should just go with a Bushmaster XM-17 Bullpup?  Its only 30 inches total with a 21 inch barrel.  I could get Kurts Kustom to cut that baby down 4 inches, and I would be whithin 3 inches of the shorty, and have a full length barrel.

Hmmmmmmmm....Opinions?

Well, if it was me I wouldn't even consider a barrel that short.

Option B, doesn't require the SBR tax, so you would be able to use it, instead of waiting for your stamp.