Well the 9mm rds fit into the .40 mag pretty good. But I dunno how well it will feed. There is a "very slight" wobble of the round from side to side. IMO, its just cheaper to buy a $33.00 9mm mag instead of risking screwing something up, lol

But if u have a .40, just buy a 9mm barrel and a 9mm clip and wohooo now u have a 9mm shield. Thats the main reason i bought mine in .40. I have another 9mm glock and I have over 1000rds of 9mm. So its cheap for me to shoot the 9mm out of my .40, because I dont have to buy any practice rounds......For personal protection, i just use the .40 rds with 180GR. The best of both words (in a sense).

PS: You can buy a 9mm barrel and a 9mm clip for about $130 shipped to ur door (give or take a few dollars). So why not do it, if u already have a .40 to convert.

Here is both of them...9mm and .40