A. Are those guns real?
B. Are those guns for real?

Jen: I got an A on my test.
C. Roy: Is it real?
D. Roy: Is it for real?
E. Roy: Are you real?
F. Roy: Are you for real?

1. Which of the sentences in each set above is/are correct?
2. Which response is/are natural in the second set?

3. What is the difference between 'real' and 'for real'? If my sentences are not good examples, could you please give an example?
4. How is C and D different in meaning from E and F?

Please advise. Thank you.
The meanings are essentially the same. 'For real' is a casual or slang expression which adds a note of doubt to the speaker's utterance.
I see they are all essentially the same in meaning. However, 'for real' adds a note of doubt as you explained.

Thank you very much for your help.