A 26" barrelled repeater has about the same OA length as a 28" barrelled O/U. The gas system in the forend adds a little weight forward, which most trap guns need.
The 390 is widely used for clay games, including trap. It has the rep of being quite reliable.
If the 26" barrel bothers you, get some nifty long extended choke tubes. Now you'd have a 27" barrel.
Use the 390 for trap. Later, you may wish to get another shotgun for trap. Lie down until the urge passes.....