RE: S & W Sigma .40 cal
They have a heavy trigger pull, you need to practice good trigger control or you will pull the shots low. That's the reason why some fo the cheap pistols are more difficult to shoot than betetr one.
Set up a target in you basement or living room, etc. Make sure the pistol is empty (triple check this and make sure there isn't any ammo in the same room.) balance a dime on the front of the slide and practice dry firing the pistol. You need to take up the trigger smoothly and evenly while keeping the target on top of the front sight. Practice doing this about 20 times a day. Honestly, it will help, but a trigger job that cuts the weight of the pull in 1/2 will help much more as well.