The archives question is out of my scope but you might find this interesting.
An online search came up with a few sites that may give you some information, though of course not like the Archives’ holdings. I did a quick online search for the 30th Division or 117th Infantry Regiment unit history but didn’t see one. You may want to check occasionally to see if one gets put online. Just recently my great-grandfather’s WWI unit history was put online and it gave me a lot of information.
Here are the links to the sites:
This unit history mentions Company C
COMPANY "B", 117TH INFANTRY, 30TH INFANTRY DIVISION - TENNESSEE NATIONAL GUARD
BY WARREN C. GILES
Knox County in World War I
30th Infantry Division site
There is a downloadable chronology of the 30th in WWI on the left side, above the photos.
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Dear Mr. Rutherford,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the series Records of the 1st Through 338th and the 559th Infantry Regiment, 1/1/1916 - 12/31/1921 in the Records of U.S. Regular Army Mobile Units (Record Group 391) that includes records of the 117th Infantry Regiment (in Boxes 1871 -1899).
These records as well as the ones you located in Record Group 120 are in the custody of the National Archives at College Park - Textual Reference (RDT2). The staff RDT2 will be pleased to make these records available to you or your representative in the Textual Research Room located 8601 Adelphi Road, College Park, MD, near the University of Maryland--College Park campus. The Textual Research Room (Room 2000) hours are 8:45 a.m. to 5:45 p.m., Monday through Friday, except legal holidays. The RDT2 consultation room hours are 8:45 a.m. to 4:00 p.m., Monday through Friday, except legal holidays. No appointment is necessary. Prior to your visit, please consult College Park websites at https://www.archives.gov/dc-metro/college-park/, https://www.archives.gov/dc-metro/self-service-copying.html, and https://www.archives.gov/research/;start/getting-started.pdf.
Please bring the following information: Record Group number, series titled and finding aid with entry number, unit of interest, and box numbers. For example -- RG 120; Records of Divisions (P 1241); 117th Infantry Regiment, 30th Division; Boxes 14 & 15.
We hope this is helpful. Best of luck with your family research!