Smokin-Guns already has a five-shot, tubular mag: the 1897 Winchester. Granted, it is pump action rather than lever action, but I imagine it is just as affective.
Also, the current dev, team is both lacking a full-time modeler and they are currently pushing to get the stand-alone (SA) out. When the SA is released, all dev work will be turned over to the community. So if you feel that Smokin-Guns needs a 1887, then I encourage you to learn how to model and implement it yourself!
Welcome to the forums, its great to see new interest in the game!
-Pard