It is a mature bear but I do not think it would weigh 300 lbs. Looking closely at the head and belly makes me think it is somewhere in the 250 - 275 lb. range. The legs look short and the ears look small on the really big bears. This is without a doubt a nice sized bear. If this is a Spring photo then this bear might add enough weight to push 300 or maybe more by late Fall. I think most fellows are a bit too optimistic when judging live bears.