The arm will close with some of the slimmer ones. I've seen the Line and similar eGo-sized RBAs with the arm closed, usually with a couple of 510 extender adapters stacked underneath them. The Bulli A2-T closed just fine on mine with no adapters
But that's the only one I've had that experience with. I use a phoenix style dripper on my Darwin, as well, but I don't close the arm at all when I am since I only drip at home. That one will probably close with a couple of adapters, I just don't like stacking adapters if I can avoid it, myself