Saying or shouting “bullshit” may be appropriate to make your strong rejection or condemnation known as, for example, if you are in an audience listening to a politician; or, if you are asked for, let’s say by a reporter, for a one sentence response to a political speech.
However, in the context of a discussion, saying “bullshit” is, at best, an expression of rejection. But since it may not be clear what is being rejected, the response could and should be: “What are you rejecting?” Is it the importance of what I am saying? Is it the truth of one of my statements? Is it my reasoning? To avoid such follow-up questions, instead of saying “bullshit,” say: what you just said is so trivial, it is not worth discussing; or, what you said is false; or, your reasoning is faulty. And each of these comments should be followed-up with reasons. This way a discussion has a chance; otherwise, saying “bullshit” may just be a conversation stopper.