what are you doing hunting a trophy or trying to save your ass?
Nothing wrong with overwhelming ordinance. Its almost a guarantee. Your carbine may or may not work cause you (nor anyone else) has ever takne a T-rex. So what if we blow it to chunks? whose waiting for the meat?
I love fantasy gunfights, we can argue all week and laugh up our sleeves at our opponents who take it seriously.
I would use salt . You put salt on the T-rex tail and it dessicates their tail and they lose balance and trip and fall into a ditch.