10mm and .40 S&W

Question
I was wondering if .40 caliber bullets can be shot out of a Glock 20 10mm auto

Answer
Yes, the projectiles are the same size.  The case lengths are different and the 10mm takes a large pistol primer (.40 takes a small. In fact, the .40 was developed by scaling down the 10mm.

The only thing you should have to change is the barrel - magazines should work.  In fact, some time ago in one of the gun rags there was a story on a guy who did just that.  He had a G20, and he got a .40 barrel and .357 SIG barrel, and would shoot it with just the barrel swap - everything else (mags, recoil spring, etc.) stayed the same.