Some 3-piece ecigs have long tubular attys (like 510) and part of the cart sticks inside the atty - the bridge that brings
juice to the heating coil is down in that atomizer tube and the cart needs to touch it with stuffing and
juice. You cannot cut off part of the cart. You can get round tip or flat tip carts.
Some 3-piece ecigs have small plain steel attys and their carts go over part of the atty. That kind of cart has a smaller recessed
juice cup and you need a paper clip to push the little juice cup up to add juice.
I usually use Boge soft-cap (standard resistance only!) cartomizers instead of atty+cart on 510. The cartomizer takes the place of both the atty and the cart. I also use a Joye 306 (standard resistance only!) on 510 and it is a tiny atty and has 306 carts that slip over the atty (and they have a small juice cup you need to push up with a paperclip to add juice)