Tight fitting, machine knit cotton undershirts had become common by the start of the Civil War.  When the Fluvanna Artillery sent a baggage wagon back to Fluvanna County to collect items from soldiers' families to help them through the winter, some such shirts likely made their way to camp.  This reproduction from South Union Mills is comfy, will keep you warm, and is useful for any impression.
Available in size Regular (will fit up to a 42 chest) or Large (will fit up to a 48 chest).