Actually...I'd have to agree with Wilson on this one. Primitive, less effective guns and guns advanced as the ones today have a massive difference. In the case of advanced guns, the pirates could very likely slaughter the merpeople. If that's what you're going for, cool, but it seems to me like you'd need to even the playing field. 

Not looking for conflict or anything. I just would really like some clarification. It would be helpful to set the scene and set boundaries.