First let me say that the issue of “Popular Truths” really bothers me.
This like the JFK shooting, the OJ trial, Pam Anderson’s marriage, J Edgar Hoovers sexual preferences, the fact that we have forgotten most of history that was inconvenient or not politically correct and the myths that are perceived as truth all really bother me.
What history you ask? Besides the Hoover myths, how about the fact that somehow we “forgot” that Christopher Columbus did not sail to America, nor did he sail for the purpose of exploration, he was collecting slaves, Columbus was a slave trader.
Believe me I can go on and on and on, I found no less than 136 errors in my daughter’s 4th grade math book and 31 errors (out and out fabrications) referring to the constitution and it’s founding in my second daughters history books.
Ok, to return to the topic at hand.
First let me say that there is nothing "wrong" with the .40, but there is nothing wrong with the 9mm when loaded with equal loadings.
If we put aside the last few thousand articles/marketing promotions you have read and ask yourself this question:
If you take the two metal projectiles, both optimized for the velocity they will be driven to and one weighs 127grains and the other weighs 155grains. We will start with projectile A, projectile A is 9mm and travels @ a velocity of 1,220fps from your barrel/gun combo. With a barrel swap in your pistol, you can use projectile B, projectile B is .40 S&W and it travels @ 1,200fps from said caliber/pistol combo.
Pistol B achieves this with an increase in recoil momentum measured at 6lbs for example. You will need 5 rounds to defend yourself all on target in a 4" group in less than two seconds. You can achieve a reduction in felt recoil of 30lbs in 2 seconds and you remember that your pistol was in fact first designed and offered for the Austrian army pistol trials in 9mm, then when the market “exploded” in the USA, your pistol was quickly bored out, given an extractor tweak and shipped as a .40. When it was designed for the 9mm round, it met the minimum service accuracy requirement that it achieves 1.5" groups @ 25 yards with your chosen ammo. As noted, after some early frame breakages the retrofit was reexamined and a an extra locking pin was inserted into the frame of pistol B, given that pistol B was a redesign from the factory, it achieves 2.2" groups from the factory.
One other factor is that the .40ammo costs you $1.50 more per box for FMJ practice ammo.
Change the environment now, you live in a rural area and you are in fact a patrol officer, just you and a 90 mile stretch of road. The last two shootings your agency had were at ranges of 26 and 120 yards when your officer came under fire from drug smugglers crossing through the desert lanes of your county. You are restricted to a Glock pistol in 9mm or .40 with either 15 or 17 rd magazines; you are issued a Winchester .12 gauge model 1300 defender with reduced recoil 00 buckshot only. You are deciding between pistols your choices are pistol A the 9mm that fires the above round with a ballistic coefficient of 0.143, you know that it delivers 341 ft-lb of energy @ 100 yards and it shoots to 3.6" of point of aim @ that distance. You will be issued two spare magazines with one fully loaded and you may "top off" your pistol for a total of 52rds.
You have pistol B that fires the above round with a ballistic of 0.165, you know it shoots to 4.0" of point of aim @ 100 yards and it delivers 357 ft-lb of energy. You will be issued two spare magazines with one fully loaded and you may "top off" your pistol for a total of 46rds.
You are expressly prohibited from carrying a second gun and you know that the last shooting was in fact 120 yards and backup is 15-20 minutes away.
Which pistol are you going to choose? What pistol really offers you a true ballistic and tactical advantage? What pistol offers you a true mathematical advantage?