This question centers more around fermentors. I would imagine you need a 20 gallon mash tun / boil kettle to make enought beer to fill a keg.
15.5 gallons is the standard volume to fill it up.
The question then becomes what do you ferment in? I have seen kegs converted to fermentors but including head space would you not need more like a 20 gallon fermentor?
What are most people using to ferment and be able to fill up a full keg when they are done?
Right now I am brewing 5.5 gallon batches fermented in 6.5 gallon carboys. Gives me 5 gallons to completly filla corney keg.
But I am not seeing alot of inexpensive stainless options for filling a 1/2bbl from the fermentor