How full to fill a barrel?

[wickerman]

When filling a barrel with beer, how full do you make it? Should it be full until over flowing? Seems like that would make an airlock fairly useless. I have a 30 gallon barrel that I'll be filling up with sour beer within the next month or so. I already have the beer fermenting, so this will be more of a secondary. Also, I haven't yet, but plan on covering the staves with a thin coat of wax, but not the heads. That should help keep it from letting too much air in, even if the top gets a little dry. This is my first barrel, so I'm basically going off of reading other's experiences.
Any info is appreciated.
Thanks,
wickerman

 

[microbusbrewery]

Ideally, filled as high as possible to minimize headspace. I have a barrel-aged pro-am beer going right now and we filled to slightly overflowing (to get any foam out), then when we removed the filler the level dropped down to just below the bottom of the bunghole. Basically the same concept as bottling, fill to the top and when you remove the bottling wand, it'll drop to the appropriate level.

I have two smaller barrels that I waxed. First one was a 10 gallon whiskey barrel where I waxed the top 3/4. Second was a 15 gallon bourbon barrel where I waxed the top half. I waxed them (probably as you read) to mimic the level of oxygen permeability you get with a 50-ish gallon barrel. So far no complaints, but I know a lot of people don't take the time to wax theirs, and they have great results. We also have a couple larger barrels for our club, and I gotta say that my small waxed barrels seem to lose less to the angels share than the 50-60 gallon barrels.