Medical records should describe the findings, examination date, treatment and subsequent course as precisely as medically appropriate. Keep medical reports, discharge notes, prescriptions and records of further treatment. Tell the treating staff about all symptoms, including those that emerge later.
A dated chronology can record where the injured person was, what happened just before the shot, which warnings they heard, when the shot was fired and who was present. Clearly distinguish personal observations from information received later. With the injured person's consent, family members can help organise records and preserve supporting documents.
If they exist, incident reports, weapon-use records, service notes, video and witness statements may assist the review. Whether such material exists, can be accessed or can be preserved must be established in the particular case. Ask a lawyer to check lawful preservation promptly, and do not alter possible evidence yourself.