Bullet velocity is one of the primary variables that go into calculating a trajectory. Achieving a sufficient and consistent muzzle velocity is a major consideration for shooters developing their own load (assembling cartridges themselves, instead of purchasing factory-produced ammunition).

Caveat: I’m not writing from a position of authority. I picked up a new hobby, and writing short articles is a way to document my learning, and encourage further study.

Factory rifles and factory ammunition achieve acceptable accuracy and precision for their intended purposes. However, competitive long range shooters desire better results, so they have customised rifles and ammunition. Dedicated shooters pay attention to a number of factors that affect bullet velocity.

Powder mass

As one might expect, the greater the mass of gunpowder in a cartridge, the more energy is directed into pushing the bullet faster. There are limitations though. A long range shooter must consider that their barrel, action, and chamber were engineered to tollerate certain pressures. They can’t simply stuff in as much gunpowder as possible, as the excess of pressure could damage their equipment, potentially causing harm. Invested shooters spend time tuning the mass of gunpowder to produce a sufficiently consistent muzzle velocity. A higher velocity isn’t necessarily better, consistency is more valuable as it helps with predictable trajectories.

Barrel length

Barrel length plays a part in producing high velocity bullets. In geneal, assuming you keep other variables the same, the shorter the barrel, the slower the bullet. If the barrel isn’t long enough, then you end up wasting gunpowder energy. The bullet reaches the end of the barrel before all the powder has finished burning, and rather than spending energy pushing the bullet in the direction you want, energy is lost in all directions (unconstrained by the barrel), and you get a fancy muzzle flash. Although, you don’t want to go too long either as it would spend too long engaged to rifling, and begin to slow down in the barrel! Barrel lengths in the range of 30-32 inches are not uncommon for competitive long range shooters.

Transonic threshold

A bullet leaves a rifle barrel at supersonic velocity. Even though the bullet is aerodynamic, and very fast-moving, it’s still relatively light, and air is not free to move through - it immediately begins slowing down due to air resistance. In the supersonic range bullet trajectory calculation is well understood, and more predictable. A threshold to be concerned about is when a bullet slows down to 1470 km/h (1340 ft/s). At this point the bullet enters the transonic range, an undesirable region for consistent trajectory calculation. In the transonic range there is air flow around the bullet at both supersonic and subsonic speeds. This separation causes unsteadiness, and increased drag, making the bullet’s flight uncertain. A long distance shooter will endeavour to develop ammunition that keeps the bullet sufficiently supersonic for the range they want to shoot.