Hell it's all so long past. I know quite a few people did in depth research on all this back when damagelogs were enabled during and after the rebalancing.

There were excell sheets and stuff to be found.

Anyhow as for the result of the OP's research: Asuming you did your experiments correctly (meaning you had no armor or resist on the target, SI was 0% and the target always was fully healed), the higher "efficiency" of some weapons indicates that there is some bug in those weapon's PvP dmg. calculation. If I remember correctly only 66% of dmg dealt by a weapon will go into PvP dmg calculation. Off of that goes shield, armor, resist in that order by a rather complicated formular (it's not simply additive).

So actually your experiment should have shown a efficiency of 66% for each weapon.

Note: I'm not sure about it completely, possibly there were different initial PvP dmg. mali for each hitzone and the headzone should be reduced to 50% PvP dmg.

Anyhow: When shooting on a runner without any shield, armor or resist every weapon should have the exact same efficiency otherwise the system is simply broken at base level already.

Another note: I again do not know for sure, but I remember that the dmg numbers shown ingame in the weapon info were off by quite a bit on some weapons/ammo types. In contrast the dmg log should show the initial DMG if I remember correctly.