I have an older American Arms (look like a Bond) in 357 mag.With 357's the lower barrel is on at 7yds. and the top is 17" high, with 38 158 the top barrel is on and the lower is 17" low so I use a mixed load, if I get it backwards they are 34" apart. I have Davis in 32ACP with Win. Silvertips it puts 4 shots in 4" (2 each barrel) at 7yds. When I go to "regulate" them, I set up large cardboard, have a table, many brand and weights of ammo, and make lots of notes (also masking tape to cover holes). On a Derringer that shoots "mixed" use a red perminent marker on a barrel and color the primers on ammo fo that barrel, also load notes only on what works is stored with the pistol. I have a 22mag High standard and a 38 Special on the same design by American that I haven't regulated yet, I'm thinking 1 to 2 hours of range time each. I have lots of Federal 40 grain "C" ammo I know not to use, I need to P/U partial boxes at the gun show. 38's, I'll try 5 to 6 varities of factory and 4 to 5 hand loads. A lot of work for an under the table gun! Ivan