I am assuming because you are mixing in the priming sugar, you are not bottling carbed beer.
I would think the beer gun would work, but why not just use the siphon?
You could use the keg as the "bucket" to hold the beer so you can mix in the sugar, then just siphon from the keg to the bottles.
Beer is like porn. You can buy it, but it's more fun to make your own.