Copied from my post at
cave-stg:
There are two states the game can be in with regard to chips: zero timer and positive timer. The timer increases when chips are generated; more so when the C-shot was used than with the A-shot.
With zero timer, all normal enemies release pink suicide bullets when destroyed. The suicide bullets may be cancelled with the laser; in fact, while holding down A-shot, the laser aura protects your ship so you are completely immune to them. The only way to generate chips in this mode is to destroy a special bullet-cancelling enemy (including mid-bosses and bosses) with the lock shot.
With positive timer, all enemies generate chips instead of suicide bullets. The value of the chips is determined by the way the special enemies are destroyed. If you kill a special enemy with n lockons (possible values: 1-4), the chip value is set to n. If, in addition to getting 4 lockons, you are pointblanking the special enemy when it is destroyed, the chip value is set to 5. In any case, the chip value cannot decrease while the timer is positive, so if the chip value is already greater than n when it would be set to n, it retains its previous value.
For the purposes of this calculation, the chip value during zero timer can be assumed to be 1.
The stage multiplier and overall counter are increased by i) collecting chips and ii) generating suicide bullets. The rate at which the displayed numbers increase is capped, so they may appear to continue increasing while the actual multiplier is static or decreasing; this does not affect scoring. The stage multiplier is 'cashed in' and decreases when enemies are destroyed with lockshot. On end of stage bosses, the overall counter assumes this function. The end of stage bonus is related to lives, bombs and the overall counter (exact formula?).
A large proportion of the points seems to come from the bosses and end of stage bonus. This is because the boss multiplier is potentially ten times as large as the stage multiplier. For instance, in my best run so far (~380mil) I think these bonuses accounted for around half the points. From watching replays, it is not clear that blue suicide bullets actually benefit scoring particularly (though obviously the no-miss that automatically triggers them does).
Blue suicide bullets, which are not cancellable by A-shot, are triggered by no missing until 240mil points. Dying after that point reverts it to pink suicide bullets. Reaching the last boss with blue suicide bullets causes it to be substantially harder (more like 2nd loop arcade difficulty). It is currently unknown how DOOM is triggered, but it doesn't depend on lives lost. Using 6 bombs or fewer has always worked for me, though even that condition has been proved to be too strong.