I've never used a 306 carto, but did a search and read up on them...others may have more advice.
306 Cartomizer
"Product Description
The 306 cartomizer features a "soft tip" for easy refilling and comfort during use. Our 306 cartomizers are rated at 2.8ohms of resistance.
Drip tips cannot be used with 306 cartomizers.
Hope that helps, or hopes someone else has more info.
Deb