i would be wearly of only 1 gallon worth of headspace. I blew the off air lock straight to the ceiling with my big chocolate stout... and on top of that id say 1/8th of a gallon of liquid from foam came out. It was a ridiculous amount of Krausen.
If you use a blow off tube, it will work... I went with a 7.9G primary now, and keep the 6.5 gallon one in the freakin shower. HAHAHAH!!!
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