A: When someone’s shoulder is hurting during a back squat, it is usually a sign of shoulder impingement (external rotation + abduction). Depending on the severity, you may want to get it looked at. Front squatting (I prefer using straps) would most likely feel better than back squatting.
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Front squat using straps
You could also try squatting with a Rackable Cambered Squat Bar . These are options to work around the pain and still create a training effect. But don’t neglect some good old soft tissue work. I'd bet your shoulder has some barnicles growing in it :-) ......The Foam Roller should be your best friend for a while.