That's not solder, that area was brazed. Judging by the shape of it, and this is just a guess, it looks like it developed a crack in the spacer between the barrels and was brazed back whole again. It doesn't look new, so it probably was a successful repair. I’d not worry too much about it if you are going to use it for shooting, but if you are keeping it as a collector, it will depreciate the value some. Just keep an eye on it. I would think that it is o.k. to shoot.