Update for the Dogpound: Though I have a "classic core" model, I got an old Zap stem from Dave before he introduced the new core and his own stems. And I use it with my classic core and I LIKE IT!
I had complained in this thread before how I had not been roasting stems to the color of my liking. I soon discovered the fault of my "stem packing." The silicone stems are amazing for loads greater than maybe .05 grams, but weren't giving me that "almost as dark as coffee" dark roast that I was looking for for really small loads. A good roast not only depends on the heat core temperature, but have everything to do with airflow. And restricting some airflow is good for high temperature roasting. But how do you restrict airflow when the amount of material in your stem doesn't even cover the screen?
Fast forward to the present and the new core (and nylon stems) has resolved this issue. By narrowing the diameter of the roasting stem (5/16 ID at the stainless steel tip), the same amount of material gets "packed tighter" and roasted darker.
But those of us with classic cores? The nylon stems seem to work damn well! Just hold stem tip directly against heat port screen (which, on my classic core, is about 1-2mm recessed from the top of the heat port walls) and draw. NOTE- the downside of using a classic core Dog to do this is that you need two hands...one to hold the stem and one to hold the Dog.
I'm getting super dark roasts with VERY VERY little material. In fact, I actually combusted once using this method (via the wallwart running HOT, which it seems to do, particularly in the evenings), so watch your roast, playa!