In a quandary

[Solus]

.40S&W is just a down loaded 10 mm. Because 10 mm had to much recoil for FBI.
That's why they call it "short and weak".

True.  The .40S&W also fits in a smaller frame making it usable by folks with smaller hand sizes and easier to conceal. 

Further, the .40S&W meets the FBI criteria for an "effective" round where the 10mm does not....because it over penetrates, which leads to the round exiting the target and causing collateral damage.  Also, when it does exit, all that extra energy is not delivered to the target and there is reduced control and follow up shot speed.  There is also the additional battering the weapon will take.