bottle and keg the same batch

I have a 5 gallon batch of porter that I'd like to bottle half of and stick the other half in one of my 5 gallon corny kegs. Has anyone done this? Are there potential problems with leaving that much empty space in my keg while it carbonates? I'm assuming I'll have to use CO2 to purge the oxygen after I keg it.

Or am I just better off buying a 2-3 gallon keg for this method?

Thanks!

 

Other than the fact that you're using more gas I don't think it will be a problem. The empty headspace will be taken up by the co2 and as long as it's at the proper PSI the beer doesn't know how much headspace there is. Let me finish this by saying I am no kegging expert.