Linked from
The 9 pages that link to Internal Ballistics, each with the reason it gives.
GunpowderRelated: It describes how confined gunpowder combustion accelerates a projectile.
Muzzle velocityNarrower topic: It describes the processes that generate muzzle velocity.
BlowgunRelated: It covers pressure and friction while the dart moves through the tube.
GunsRelated: It explains how expanding propellant gases accelerate a projectile through the barrel.