I Didn't Know This, .40 S&W through 10mm G20

[BPTactical]

Burt what could happen?

Oh, other than a catastrophic barrel failure not much

You are going to have the difference between the end of the 40 case and the end of the 10mm chamber that will foul and lead up. Worst case it will shave some of the 40 projectile on the crisp edge which is the end of the 10mm chamber. Do it enough or that one round that is "just right" and it can cause a pressure spike which is on the list of bad things.
Not to mention the erosion on the 10mm throat.
Where the worst potential for an issue is when NumbNut loads a 10mm unwittingly and pulls the trigger. Now that round is crowded into the chamber along with all of the fouling etc.
Pressure spike.

Again, only a stupid ass would do such.
We aren't stupid asses here.

And Bailey is spot on, it would be very hard on the extractor.