If you look closely at the typical Glock 40's brass, you'll see a bulge, like it's pregnant. This is the place where the feed ramp leads into the chamber. The cartridge settles into the chamber properly, but that part of the case above the ramp is not supported, and when the brass stretches, it leaves a bulge.
The severety of the bulge is dependant upon the pressure the case has placed on it and the thickness and overall quality fo the brass. Some brass is less elastic than others due to it's metallugy.
Now, as to loading.... I have personally loaded brass that was fired in a glock and had a barely noticible bulge. I would never load a casing that had a bulge as bad as some picts that can be found on the net. If starting out with the 40, it's best to scrap all Glock 40 brass for safety. Glock brass can be ID'd by the rectangular impression around the firing pin strike on the primer.
Now, some after-market Glock barrels are more, or fully supported. THe best way to check a Glock barrel is to remove the barrel from the gun and drop a round into the chamber on it. If you can see more than a millimeter of the case wall where the feed-ramp meets the chamber, it's not fully supported.