I intended to order a 510....I actually ordered a 306... An avid vaper Cisco spec HR306 3 ohm atomizer to be exact. Pulled it out of the package, not realizing my error, and thought well hey! What's that one the end? Looks like a cartomizer cap! So I tried to pry it off. Few dings to the center hole later I realize that what I have is a new species to my world....and I know nothing at all about it. They say I can send it back....but I was REALLY looking forward to vaping off of a decent atty today. Who knows what about these?