Clorox recommends a ration of 1 tablespoon to 1 gallon of water, or a ration of 1 part bleach to 256 parts water for disinfecting. When cleaning stock tanks, I dump a gallon of bleach into 125 gallons of water, or a ration of 1:125. Usually clean the sides of the tank and bottom by sticking a sponge into the water with my hand and swirling it against the sides and bottom. I end up smelling of bleach, prefer not to splash myself, notice sort of a greasy feel to the water, but other then that, no harm, no foul.
If you're talking a standard barrel, you're talking 55 gallons. (Sixty if you stuff it to the brim.) If you dump a gallon in, this is about double what I work with or four and a half times Clorox's recommendation. Might turn your hair white but probably not, will feel a bit funky, will stink a lot, but probably not that harmful.
Here's the
Material Safety Data Sheet for Clorox bleach. It's a PDF file that will download onto your computer. Even a 100% mix seems to only cause minor irritation.
Best of luck,
Jim Clark-Dawe