Strange.....the "Bob" is nothing more than a brass cartridge of certain dimensions.....and possibly there's a 1-12 twist but I've never had any problems with my 1-10 twist M-70 Bob and Particians.....Try the 100 grainers and see if there is an improvement.....also check the twist rate.....if it's 1-10 or faster it should handle the 120 grainers quite well.