And the iStick isn't another quality made mod built with good engineering either... so it's not a good example to make a point with...
All of my Subtanks from the original, to the Mini and Nano fit flush on my VS DNA 30w, iPV Mini v2 70w, a lower quality Amod ZNA clone 30w ( manual adj ), and even my crappy Smok XPro BT50w ( yeah, I don't care for Smok products )
I will say in defense of certain mods... the 510 standard only has the size of the threading defined, the length was never standardized which is ultimately the underlying cause of this issue to start with. Many manufacturers have seemed to follow a set of standards they defined themselves... but when one companies device uses different standards than another device made by the same company... then their engineering is all that we can call out which is the case with Smok if it is working on my mod by them and not on the one you have.