OK, I know the following rules seem to apply but I need some clarification.  I know a rifle must be 26" long.  I know my preban AR fully collapsed with my 16" upper is 32.5 inches long.  So why can't I mount an 11.5 inch barrel to my preban rifle unless the flash hider is permanantly attached?  My math gives me a total length of 27" for this combo. Please explain this to me.  Thanks.

Damn, I didn't think it took me that long to type this...BTT

Its not the overall length thats the problem here, its the barrel length.  The barrel must be at least 16", otherwise it is a short barreled rifle (regulated under the National Firearms Act) and must be registered as such.

Title 18 contains the statutory definition...

18 USC 921(a)(8) The term ''short-barreled rifle'' means a rifle having one or more barrels less than sixteen inches in length and any weapon made from a rifle (whether by alteration, modification, or otherwise) if such weapon, as modified, has an overall length of less than twenty-six inches.

Ohhh,  Two different laws not two different interpretations of the same law.  Much clearer now thanks.