The secret to doing this correctly is LEANING FORWARD!!! Lean forward and your ROM will significantly increase because you will not have to maneuver the bar around your buttocks and do the squiggly-wiggly maneuver (been there, done that). Try leaning forward more, rotate your feet so your toes are pointing slightly out, and go for it with less weight than normal. Once you get the hang of it, the barbell hack squat is a HELL of a movement. It's like a front squat mixed in with a deadlift.