I think the gun itself is the same. The 6850 comes with 2 Ni-Cad batteries and the 6855 comes with one Lit-Ion battery. I personally like the Ni-Cad batteries better. Everything I have with Lit-Ion really **** in cold weather and I don't really notice that much advandtage in warm weather. The only real advantage is size. Not really a big deal when using a tool the size of a 6850.