The Volt cartos are supposed to be rated at 2.5 ohms. However, there will be variations from carto to carto. The batteries haven't been used that much yet, so they still have a way to go. They still hold a charge quite nicely.
I got my Vortex version 2 cartos. In a word, wow! These are rated between 2.4-2.7 ohms and perform beautifully. No leaks at all. The flavor is very clean and comes through nicely. The throat hit is amazing. The vapor production is way better as well. Now my Volt is kicking ***. I'll post a review after I have tried them a few days, but so far I'm very satisfied.