From what I understand: 1. New gas block and piston. Gas block no longer threaded. The new one is easier to clean. I've never had an issue with mine. 2. Color options. Gen 1 were made in black only. 3. No optics. Gen 1 came with a Sig red dot. 4. Price. At the same time, Sig introduced two other models, M400 and the 716.