There are three separate files containing his service in the Veterans Reserve Corps., see:

Augustus B. Sweeney, 2nd Lieutenant, 20th Company, 2nd Battalion, Veteran Reserve Corps.

Augustus B. Sweeney, 2nd Lieutenant, Company ?, 8th Veteran Reserve Corps.

Augustus B. Sweeney, 2nd Lieutenant, Company G, 15th Veteran Reserve Corps.

Index to Compiled Service Records of Volunteer Union Soldiers Who Served in the Veteran Reserve Corps

I believe these records may only be found at the National Archives in Washington, D. C.