I use mine for beer and wine....the beer will retain it's carbonation indefintely. It's really no different than a counter pressure filler, all the counter pressure device does is prevent excess foaming by equalizing pressure....with the Beergun, you transfer using a lower pressure, about 5psi, this inhibits foaming.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Bourbon Barrel Imperial Stout
On Deck: German Lager