Armalite def-10 handguard

[Everydayjoe]

Not a total noob but new enough to the AR-10 platform to seek input. 
Im looking at a new Armalite Def-10. It comes with an 8”  A-2 handguard and would like to upgrade to something with rails. From what I measure I have up to 15” between the flash hider and the end of the barrel nut. Is there any reason I can’t get a hand guard longer than the stock 8”? 
I’d like to know what limitations I’d have before I purchase.