use a blow off tube though. you'll likely blow krausen thru the airlock/blow off tube...without one you'll just have a mess around the carboy.
6.5gal food grade buckets are cheap. i primary in that, secondary in a carboy that's only 5gal. works fine.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10