Years ago when i had Xd pistols in 9 and .40, i discovered that i could interchange magazines- and the .40 would run with 9 mags, and vice versa. ( i didnt shoot a lot this way, but in my limited sample it worked). Now with glocks- i find with factory mags the same swap is not very reliable. The Magpul glock mags though- made for 9, do seem to run .40’s just fine ( so far). I’ve also found the 9 “ conversion barrels” for the .40’s ( with proper mag) run with no problem, no other parts need to be swapped. In todays world where what ammo is available may change month to month- having pistols that will run more than one caliber might be useful.
