645 slides project nicely using standard 6X6 or 6X7 projectors. The problem is finding the appropriate mounts (and the projector). Labs used to mount them for 6X6 projectors unless you emphatically told them not to. A few may still do it if you emphatically tell the to do it. Hand mounting slides is not difficult. Although there are lots of special tools that can make things a lot quicker and easier, you can do a quite adequate job with scissors and a common iron.