What is the history of a J A Loomis Damascus Double Barrel?

I can't find Loomis in any of my references. If it is a utility grade, double barrel, probably with side hammers, I will guess that you will discover Belgian proof marks under the barrels. Look for an oval surrounding the letters ELG or a mark that looks like the Washington Monument. Several million similar guns were imported to the US c. 1880-1915, marked with several hundred trade names. If you know the location where it was acquired c. 1900, check the old business records for a Loomis Hardware or Loomis Mercantile store.