If you're shooting a fixed sight handgun and your shots are going low, you should look at your trigger pull before you blame the gun.
are you attempting to counteract the recoil by trying to push the front of the gun back down? this will cause your shots to go low.
get a small sandbag or something somewhat soft and relatively heavy (a folded up jacket will work) and hold the gun on it. set your sight picture and pull the trigger. do this repeatedly so you get a shot group. if the shots are all going where your sights are pointed, the problem is your trigger pull. if it is still going low, look at the ammo. In 9mm at least, this can happen on some guns with 115g ammo. with Sigs at least, this is common. they test fire the guns using 124g ammo, which acts differently than the 115g that you would normally use for target practice. How would you go about fixing this while still using your target ammo? Use a taller rear sight. On a fixed sight handgun, this isn't an option, so your choice is to either aim higher (figuring out where you need to aim in order to land dead center) or switch ammo.