I just bought a G20 compensated 10 MM and wanted advice about converting it to shoot .40 S&W
ammo .

I'm mainly interested in what barrels work best , if they can be ported to match the holes in the factory slide , and if I can use the 10 MM mags for .40 S&W ammo.  

Thanks ,
Smiley

Not sure if it's possible, the mags are different, .40 rounds would fit in a 10mm mag, but since 10mm is longer, you would likely have serious problems iwth magazine jams/FTF's.  I don't know of any .40 barrels for a G20, not sure if you could just drop in a .40 cal barrel.  Maybe somebody would make a custom barrel ?  As for the mags, you would want to find some way to modify them so the .40 caliber rounds are held in the correct position in the mag (instead of being able to slide backwards and not feed correctly).  

Quoted: I'm mainly interested in what barrels work best...

KKM makes a nice 40S&W barrel for the G20 available at Lone Wolf, item KKM-G20D1

Quoted: ....if they can be ported to match the holes in the factory slide , and if I can use the 10 MM mags for .40 S&W ammo.

Yes and yes.