Here's a video taken tonight of me doing a 405 pound bottom-position squat. The BP squat is one of the best exercises you can ever do for building massive strength. The problem is that most lifters do them incorrectly. Because they lack either proper form or flexibility (or both), they end up doing partial squats.
This is how the exercise should be done. Notice foot placement and bar placement. It's tough to get low on a BP squat if the bar is too high and/or the stance is too narrow.