Just to clarify...if you cap snaps over the top of the bottle(ie 10ml childproof style) then you need the 1/4" tubing. If it snaps down into the tube(ie 6ml non-childproof style) then you need the 3/16" tubing. The 1/4" tubing is far more available from any hardware store...so unless you just have to have "exactly" 6ml's, I'd go with the 5ml, 10ml or 30ml childproof from MadVapes or the 5ml bottles from TotallyWicked. These all work with 1/4" tubing and seal good and tight, with no oring or modification required. Just cut your tubing to length, insert into cap and you're good to go.