I can not tell the difference. Maybe the K40 spring is a little stiffer? I have a Tanfoglio stock 2 in 9mm and a Tanfoglio witness match in 40. Went to the range the other day and tried the K40 mag loaded with 9mm in my stock 2 and it fired 100%. I know the CZ full size 9/40 mags are one and the same. I have not heard that about the K9 and K40 mags. Does anyone have any thoughts on this?