I have both Glock 33 and 27's. I understand that I can interchange the barrels and shoot 40 cal thru the 33 and 357 sig thru the 27 using the original magazines and slides. . Recently at a gun show in Louisville Ky I met a "certified Glock aromorer" who sells after market barrels for about $90. Are these barrels OK ? He also told me that I could replace the barrel on either the 33 or 27 with a 9mm barrel using the original slide and magazines. My gun people locally do not believe this. Is it true ?