An idea I had so that I can keep my suspension of disbelief is that was actually a rail from a railgun.
The flash can also be explained as it being a hybrid conventional/railgun propulsion, where it's initially accelerated like a normal bullet through burning gunpowder, and then the electromagnetic propulsion gives it a boost.
Reason for doing it that way; longer lifespan of the rails, lower energy requirements for the electromagnetic propulsion. In case you feel like explaining it even further if you go with my idea.