There are a few gun combinations that exist in the game, but are unavailable due to the neccessary guns not yet being implemented. For future reference when these guns might be fully implemented into the game, here is the full list of combinations as of version 1.0.9:
New Recipies v2.0+[edit source]
Added a M1911 and Glacier, got a Frost Giant... doesn't feel like coincidence. Probably new hardcoded recipes. Needs testing.