Anywho, guns... I'm still thinking that Kagome's shaped explosive seals are probably the right answer for our propellant. The seals are small enough to fit into a ring, so they're pretty small and could fit in the end of a gun barrel somehow. I'm not sure how small we can make our other seals that propel things. We would probably need Kagome to adjust the shape and force of the explosion, but I would think Kagome could do that since he invented the seal in the first place.
From there we just need to make a good barrel and loading mechanism. We don't strictly need a modern gun barrel, it just needs to be good enough to shoot somewhat accurately.