What do you mean by bottling barrel? Are you using a barrel to bottle them all? or using a barrel and then wanting to bottle just a few of them?
If you're batch priming and bottling then you want to work out how much sugar you need for all of the bottles (6g per litre seems to be the standard for ales, though i usually do about 2.5g/L)
Boil the sugar in a little bit of water for 10-15mins (I do this to sterilise the sugar though many people miss this step out) Cool it a little and add it to your sanitised bottling bucket.
Siphon your beer onto the sugar water with minimal splashing.
Then bottle. I wouldn't leave it any amount of time between priming and bottling.
If you're using a plastic barrel to store your beer then someone else will have to give you some advice as i only use bottles.