Having a different physical form still doesn't mean its nothing like it's predecessor.*rolls eyes*
It has a totally different physical form to Futachimaru. It'd make more sense as either the top evo of a line where Futachimaru is at the bottom and there's a mid-way between the two for the middle evo, or as, say, a fourth evo, where there's the same mid-way evo between Futachimaru and Daikenki, but still with Oshawott at the bottom.
After all, just look at Remoraid. It evolves into an Octopus despite being a fish! Of course, the change in form is clearly not the focus but the theme they both share (both are based on projectile weapons).