0 - current position.
1 - position from eyes (weapon aiming).
2 - end position from player position.
3 - end position from eyes (hit point for weapon).
4 - position of last bullet hit (only CS).
That what you're looking for? granted, it won't be 100 units, but you could trim it down by using mode 1 and 3. Though this will raise your position a bit because its from the eyes, not the ground/feet.
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What an elegant solution to a problem that doesn't need solving....