Weight of ammunition matters more in some scenarios than in others.
One of the main advantages of the US military switching from 7.62mm NATO to 5.56mm NATO was the tremendous weight saving. A soldier could carry many more rounds of the smaller caliber ammunition with the same weight load.
For personal carry, my usual load-out is two spare single-stack magazines or one double-stack. My body can’t tell the difference between two magazines each holding eight rounds of .45 ACP, or one holding fifteen to nineteen rounds of 9mm Luger.
We have to adapt to changing times. It used to be when I flew to the West Coast to teach in Washington State or California, I could simply buy ammo there. In Washington, it has become ridiculously expensive, and in California it is now impossible: to buy ammo there you need to be a resident of that state with a specific state-issued license to purchase ammo. We can, however, enter the state with our own ammo for our own use.
Since I live on the East Coast I fly to those states. As a general rule, declared ammunition in checked baggage can weigh no more than eleven pounds. For that reason, I don’t take .45s to teach there anymore.
A box of fifty 230 grain .45 ACP cartridges weighs 37.20 ounces. A box of Federal’s American Eagle 9mm 115 grain full metal jacket 9mm weighs 21.80 ounces. Do the math.
Of late, I’ve been using CCI Blazer aluminum case 115 grain 9mm, for one reason: it weighs less – 18.20 ounces for a box of fifty – so I can bring a little more. On my most recent trip to the Pacific Northwest I was in three states, two of which have ten-round magazine limits, so my pistol was a Springfield Ronin 1911 in “lightweight Commander” format, with ten-round Wilson Combat magazines, and the training ammo was 9mm aluminum Blazer.
The ghost of Jeff Cooper, the high priest of the .45, need not fear for my safety in going down to 9mm. My preferred carry load in that gun is the Winchester Ranger-T 127 grain +P+, rated for 1250 foot-seconds out of a 4” barrel. (The Ronin’s barrel is 4.25”.) That’s about the same as the legendary 125 grain .357 Magnum out of a short (2.0-2.5”) barrel if not a little more.
The blue states will continue to pass stupid laws. Those of us who live by Logic will continue to “improvise, adapt, and overcome.”