I don't know if there is a word for a ballistic projectile with buoyancy, but feedback from the weapons in my dense atmosphere suggests that I should consider changing the shape of my projectile, and possibly consider propellant as well, making it a torpedo.
However, I am considering instead to harden the tip, evacuate the core, and render the projectile both aerodynamic and somewhat buoyant to improve range. Torpedoes are cool, and may be used in limited capacity, but everyone needs a rapid loading low-cost way to lay cover fire at a minimum.
Buoyancy is a function of relative mass to volume compared to the suspension fluid, and this does mean reducing the mass. Given an equal firing charge, this gives it vastly greater muzzle velocity.
Q: Can a buoyant ballistic projectile deliver equivalent kinetic energy at an equal lateral range than a solid equivalent?
Same weapon bore, same boat-tailed cone-nose projectile, but the core evacuated to give less weight. The suspension fluid is the same air at a density of 65kg/m$^3$ from my alien world.
The control round would be a 36-lb iron shot from a 450 caliber 12-foot cannon, using an 18-lb black powder charge. The test round would have 70% of the weight of the control due to an evacuated core. The hypothesis here is that a similar kinetic impact can reach the same lateral range in a shorter time, which may improve accuracy to boot.
note: A 650 caliber shot would be a cannonball, so the smaller bore allows shaping.
Buoyancy numbers are:
- Air = 65 kg/m$^3$
- Control round = 7508 kg/m$^3$
- Evacuated round = 5255 kg/m$^3$
Assume control surfaces for tumbling