This is Sniper’s right thumb I’ve seen in various sfm photos. Did he hurt his thumb or something? His thumbnail is all black but the rest of his fingers/nails are fine looking.
His sniper rifle has a manually operated bolt action on the right-hand side. The bruise (a subungual haematoma) is caused by trying to reload too fast (too many people wanting him dead on the battlefield?) and getting his thumb caught in the sliding mechanism.