Noise-Induced Hearing Loss (NIHL): This is the most likely diagnosis given the patient's history of exposure to a loud noise (gun discharge) close to his left ear, which is a common cause of NIHL. The symptoms of hearing difficulties and tinnitus are consistent with this diagnosis.

Acoustic Trauma: Similar to NIHL, acoustic trauma can occur after exposure to a loud noise, causing damage to the middle ear or inner ear structures. The patient's symptoms and history are consistent with this diagnosis.
Middle Ear Damage: The patient's exposure to the loud noise could have caused damage to the middle ear, leading to hearing difficulties and tinnitus.
Cochlear Damage: Damage to the cochlea, which is responsible for sound processing, could also be a cause of the patient's symptoms.

Perforated Eardrum: Although the patient denies pain, a perforated eardrum is a potentially serious condition that could be caused by the loud noise exposure. If left untreated, it could lead to further complications.
Ototoxicity: If the patient has been exposed to other loud noises or certain medications, ototoxicity could be a cause of his symptoms. This condition can be serious and potentially lead to permanent hearing loss.
Vestibular or Balance Disorders: The patient's symptoms could be related to a vestibular or balance disorder, which could be caused by the loud noise exposure. This diagnosis is important to consider to rule out any potential balance or equilibrium issues.

Autoimmune Inner Ear Disease: This is a rare condition in which the immune system attacks the inner ear, causing hearing loss and tinnitus.
Otosclerosis: A rare condition in which abnormal bone growth in the middle ear causes hearing loss.
Meniere's Disease: A rare disorder of the inner ear that causes vertigo, tinnitus, hearing loss, and ear fullness. Although the patient does not report vertigo or ear fullness, this diagnosis is still worth considering.