I had some excellent oatmeal stout from another homebrewer at a party a couple of years ago. It came out of a screwtop growler. The brewer used melted wax around the cap to get a better seal. I'm not sure if the wax actually did anything or not, but maybe its worth a shot.
I've got a few screwtop growlers from a brewpub I'd be into using if it works. I dont know much about using screwcaps vs mechanically fastened caps. I mean, there has to be some sort of a change happening within the bottle that puts pressure on the cap, creating a seal, kind of like when you can vegetables -the heat creates a vacuum seal on a regular screw cap. I guess the carbonation process in beer might form a similar sort of atmospheric seal.
I'm sure someone here can explain how this happens.
let me know how it works