Oh, I should say here, I got two BLACK BLU EPacks and two WHITE BLU EPacks, and it was kinda funny when I got the first white one, the holes inside were a tad wider then in the black epacks, even though otherwise they were identical other than the color, and the BLU/Volcano atomizers with their mouthpieces were a bit loose when I put them in the white epacks.
But then I got the 306 cartos and, they are a bit wider than the atomizer mouthpieces and, they fit real good into the white epacks and were tight in the black epacks. Now, I started using 306 drip tips and, they are in between the other two as far as size but are loose in the white epacks and thus I got them in the black epacks.
Bottom line, the Boge 306 cartos might be tight in the BLU epacks depending on the color you have and other factors, but they will still fit, unlike the 510s, which won't fit at all.