In another thread, I mentioned that I thought that stuff degrading accuracy boiled down to three variables about the bullet. Bullet parameter variables effecting accuracy and what their related variables causes each one to vary are:
Muzzle velocity
* powder; speed, charge weight, pressure curve
____ too slow a powder causes inconsistant pressure curve shapes
____ too fast a powder spikes early slamming bullet into rifling
____ too much or too little makes irregular pressure curves
* bore/groove diameter vs. bullet diameter
____ groove diameter larger than bullet diameter typically hurts accuracy
____ uneven diameter causes irregular pressure; distorts bullets
* bullet release force from case neck, entry into rifling parameters
____ high release force has greater spread, increases pressure
____ crooked and hard entry distorts bullets
* primer characteristics; seating, firing, heat output, firing pin impact
____ shallow seated, lightly struck primers fire irregularly
____ insufficient dented primers fire irregularly; pin protrusion less than .050"
* rifle movement in recoil during barrel time
____ irregular when rifle’s held different amounts against shooter
Ballistic coefficient
* bullet shape; amount distorted by ammo assembly and entry into rifling
____ fast, hard entry into rifling distorts them
____ oversize groove diameter lets bullets upset irregular
____ tight, sharp edged case mouth scrapes jacket irregularly
* bullet balance; manufacturing tolerances, jacket damage from rifling
____ non-uniform jacket thickness, off center core; unbalances bullets
____ deformed heel lets gas escape irregularly upon exit, upsets bullet
* bullet stability; spun at correct rpm’s to keep spin axis parallel with trajectory
____ spun too fast tends to keep them nose up; BC goes down at longer ranges
____ spun too slow doesn’t stabilize them, BC goes up
Direction out the muzzle
* muzzle axis relative to line of sight repeatability upon firing and on bullet exit
____ shooter moves the rifle after round fires and before it exits
____ shooter’s position not repeatable from shot to shot; bullet exits different directions
* barreled action repeatability in vibration amplitude and muzzle axis angle on exit
____ something interferes with its natural resonant frequency and amount of vibration
____ ammo doesn’t shoot bullet out at the right time in the barrel’s whip angles
* barrel time; time bullet takes to go from case mouth to out the muzzle
____ bullet exits when muzzle axis is on down swing or around exact top or bottom
* bullet balance; manufacturing tolerances, jacket damage from rifling
____ unbalanced bullets jump off muzzle axis upon exit
____ fouled, rough bore scrapes off jacket irregularly
I may have forgot one or more, but after a day or so of thinking and organizing, the above's my "best shot" to get it right.
Muzzle velocity
* powder; speed, charge weight, pressure curve
____ too slow a powder causes inconsistant pressure curve shapes
____ too fast a powder spikes early slamming bullet into rifling
____ too much or too little makes irregular pressure curves
* bore/groove diameter vs. bullet diameter
____ groove diameter larger than bullet diameter typically hurts accuracy
____ uneven diameter causes irregular pressure; distorts bullets
* bullet release force from case neck, entry into rifling parameters
____ high release force has greater spread, increases pressure
____ crooked and hard entry distorts bullets
* primer characteristics; seating, firing, heat output, firing pin impact
____ shallow seated, lightly struck primers fire irregularly
____ insufficient dented primers fire irregularly; pin protrusion less than .050"
* rifle movement in recoil during barrel time
____ irregular when rifle’s held different amounts against shooter
Ballistic coefficient
* bullet shape; amount distorted by ammo assembly and entry into rifling
____ fast, hard entry into rifling distorts them
____ oversize groove diameter lets bullets upset irregular
____ tight, sharp edged case mouth scrapes jacket irregularly
* bullet balance; manufacturing tolerances, jacket damage from rifling
____ non-uniform jacket thickness, off center core; unbalances bullets
____ deformed heel lets gas escape irregularly upon exit, upsets bullet
* bullet stability; spun at correct rpm’s to keep spin axis parallel with trajectory
____ spun too fast tends to keep them nose up; BC goes down at longer ranges
____ spun too slow doesn’t stabilize them, BC goes up
Direction out the muzzle
* muzzle axis relative to line of sight repeatability upon firing and on bullet exit
____ shooter moves the rifle after round fires and before it exits
____ shooter’s position not repeatable from shot to shot; bullet exits different directions
* barreled action repeatability in vibration amplitude and muzzle axis angle on exit
____ something interferes with its natural resonant frequency and amount of vibration
____ ammo doesn’t shoot bullet out at the right time in the barrel’s whip angles
* barrel time; time bullet takes to go from case mouth to out the muzzle
____ bullet exits when muzzle axis is on down swing or around exact top or bottom
* bullet balance; manufacturing tolerances, jacket damage from rifling
____ unbalanced bullets jump off muzzle axis upon exit
____ fouled, rough bore scrapes off jacket irregularly
I may have forgot one or more, but after a day or so of thinking and organizing, the above's my "best shot" to get it right.