This is how i do it. I have a 12 foot line going to a picnic tap. I take a bottle filler with spring tip, then slide a rubber stopper over it to plug the growler. The bottle filler fits the picnic tap perfect, so slide the bottle filler in the picnic tap. (be careful as you can break the picnic tap). Then I bleed the keg, open the picnic tap and slowly turn up the co2 till it starts flowing, very low psi i might hit 3psi at max. You will have to bleed the stopper as it fills, I just use my thumb to bleed it when needed. I have always done it this way and never gotten a flat growler. I know its more work than a beer gun, but its a hell of a lot cheaper too. I do the same method to bottle from the kegs for competitions, ect.