FD
Rotating bands
Dirt and burns cause improper sitting of the round
Tube conditioning
A cold tube offers greater resistance to projectile movement than a warm tube
Propellant residues
Burned propellant and chemical agents leave residues on the bore surface
Moisture content of propellant
Charges come in sealed aluminium foil, only remove it from the seal just before firing
Position of charges in chamber
Charges need to be flushed against the obturator spindle
Non-uniform ramming
Creates a non-airtight chamber, resulting in escaping gasses and a smaller propellant chamber
Coppering
Deposit of a thin film of copper on the bore, resulting from friction that removes the outer surface of the band
Velocity Trends
Same lots of round with diff. MV
Ammo Lots
Mass production with diff. raw materials and machines
Weight of projectile
Projectiles have different weights, WP rounds tend to be heavier than HE
Propellant temperature
Proper storage of charges to ensure uniform prop. temp. The thermal seal of the container protects it, thus only remove when firing.
Tolerances in new weapons
Vary in parameters (size of powder chamber, interior dimensions of the bore to machine tolerances) during manufacture
Wear of tube
Wear and tear of bore due to hot gasses, chemical action and movement of projectile