Gear Rifles - design assistance requested

[Anaraxes]

Originally Posted by Varyon

would it be feasible that a vibrating gear rifle - say, because the gears aren't quite properly well-aligned - could impart spin to the projectile?

One option I forgot to list in the gauss rifle thread that seems steampunky would be the method famously used in the Whitworth rifles (both small arms and cannon) in the late 19th century (American Civil War era, though Whitworth was English). Hexagonal barrel with a hexagonal projectile to match. (The small arms had hexagonal cross sections; the cannons had projectiles that were also twisted, so they look spiral-shaped.) The orientation of the hexagon rotates down the barrel. So, no grooved rifling that bites into the bullet; instead, the has flat surfaces that match the barrel all the way around, rotating as it moves down the barrel because the barrel itself twists. Your plan started with an elongated bore to have room for the chains; rotate that all the way down the barrel.

The Whitworth small rifles would also fire cylindrical bullets if they were made of soft lead. Polygonal rifling without the fancy ammo has been used in a lot of weapons, including fairly modern ones.

Anecdotes say that the Whitworth cannon projectiles made a distinctive and peculiar screech as they flew through the air, which also might be an appealing detail just for color.