i had tried doing front squats on a smith and i couldn't do them. i had no freedom and was forced to stay on the smith machines fixed range. so i tried doing them with regular barbell and tried a couple of variations. i finally settled on doing them the way Arnold used to do them. with the bar resting on the top of your front delts with your arms crossed across the bar.
same as with a barbell but put your feet a couple inches in FRONT of the smith bar. A good finishing move for the quads but I wouldnt use them exclusively - SQUAT!