TheGopper + Dr_Beanie, it would seem that you are the most skilled players of The Seal Hunter 1.2 that I've ever heard of! Congratulations!
Me and asker have indeed tried to make the magnum (and MP5) viable throughout the game, if you're just skilled enough. The magnum damage actually increases over the levels, to make sure that it always kills an activist on a headshot, as well as a penguin. From our experience, activists + penguins + seal cubs cause about 75% or more of losses throughout the game.
The turtles were initially designed to be just about unbeatable with dual M4A1 due to the their annoying, cowardly behavior but we kinda loosened them up before release to allow more techs to be viable. Magnum to dual M4A1 should give pretty decent highscores, no?