Dear Ms. Coffin,
Thank you for posting your request on History Hub!
The National Archives has custody of many records relating to military service in the United States ranging from 1775 through 2013. There is no master index listing all military service files, nor is it possible to request that we send all copies of all military files for service members with a given surname.
However, it is possible to research the military service of specific individuals. Please review our pages Access to Military Service and Pension Records, Research in Military Records, Start Your Genealogy Research, and Veterans’ Service Records for more information. Roughly speaking, most pre-World War I military records are located at the National Archives in Washington, DC. Most Official Military Personnel Files from World War I onward are located at the National Personnel Records Center, in St. Louis, Missouri.
A limited number of service records have been digitized and are available through the National Archives Catalog. For example, we searched the Compiled Service Records of Soldiers Who Served in the American Army During the Revolutionary War in the War Department Collection of Revolutionary War Records (Record Group 93) for “Coffin” with 552 results. We also searched Indexes to the Carded Records of Soldiers Who Served in Volunteer Organizations During the Civil War in the Records of the Adjutant General's Office (Record Group 94) for “Coffin” with 443 results. In each case, there may be additional records in those series with those names that are not yet available online.
Some records relating to military service from the National Archives and other institutions have been digitized and made available online through our digitization partners. Websites such as Ancestry, Fold3 and FamilySearch allow you to search a broad range of military records by name. These websites can be used for free at any NARA location. Some libraries also provide free access to such websites to their patrons.
For the Civil War, you can search the National Park Service's Civil War Soldiers and Sailors System. They list 576 results for “Coffin.”
If you have further questions about researching military service for specific individuals, time periods, or branches of service, please ask them here on History Hub, or submit them through our contact form.
We hope this information is helpful. Best of luck with your family research!