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Dear Robbin Dawson,
Thank you for contacting the History Hub!
If your ancestor was dishonorably discharged, he probably had a court martial proceeding that decided his discharge. These are in hard copy at the National Archives in Washington, DC and the index to access them is here as well. Luckily, the index is available on microfilm. You can contact the DC reference team at email@example.com with as much information as you have - they may be able to look up your ancestor’s name in the index and see if he has associated records, and even guide you to the next step of your research. You may also be able to access the M1105 index microfilm at another NARA location near you.
Here is more information about the records you need from an article on NARA’s website:
Court-martial records are a great source of information not only for a particular individual but also for providing insights into the trials and tribulations faced by soldiers. Records related to the proceedings of U.S. Army courts-martial, courts of inquiry, and military commissions can be found in Record Group 153, Records of the Judge Advocate General (Army). To find an individual's case file, first consult National Archives Microfilm Publication M1105, Registers of the Records of the Proceedings of the U.S. Army General Courts-martial, 1809–1890. The registers direct you to an alphanumeric series of case file numbers. The case files are located in RG 153, entry 15. Only a few select cases have been reproduced on microfilm.”
We wish you all the best with your research.