Yes you can accidentally load a 9mm or 357 Sig cartridge into a 40 mag. Don't kid yourself into thinking you will catch the mistake, I guarantee at a glance you will miss it. (Been there done that)
Yes we have test fired 9 & 357 out of 40 barrels.
Yes we have test fired 9 out of 357 barrels.
To date: We have not encountered any catastrophic failures in our test barrels but that don't mean there is not a penalty to pay down the road.
Beat the odds of having a real bad range day: Use the correct magazine for the ammunition!
*The 40 cartridge will not chamber into a 9 or 357 barrel so you are safe here. If it did chamber it would blow like a grenade.