In any case, I don't know what's up with sap. Someone with subguild sap
shouldn't be able to hit reliably, but I haven't used it much myself. From looking at the code, it appears that the chance to hit is identical to that of backstab when skill levels are similar, but the highest anyone gets sap is 80 while assassin backstab goes to 90. And subguild sap generally caps at journeyman, so like 40ish. There should be no reason that some ranger/thug can consistently KO people with sap, but who the fuck knows what goes on in the code.
According to the code, sap's damage calculation is as follows:
dam =
dice(wielded->obj_flags.value[1],
wielded->obj_flags.value[2]) + wielded->obj_flags.value[0];
if (ch->skills[SKILL_SAP])
dam *= (1 + (ch->skills[SKILL_SAP]->learned / 10));
dam += str_app[GET_STR(ch)].todam;
if (wpos == ES)
dam = MAX(1, (dam / 2));
else if (wpos == ETWO)
dam += number(3, MAX(str_app[GET_STR(ch)].todam, 4));
The way I interpret it, it rolls your weapon's damage, multiplies it by 1+(sapskill/10), adds your strength bonus, then adds or subtracts some if you're etwoing or were only ESing. Let's say you have exceptional human strength (+3) and are using a bigass fucking maul with like 2d5 damage, and you rolled 7 on the dice. This should give a final damage output of (7x9)+3+(3 or 4), or 69-70. Even if you rolled a perfect 10, it's still just 96-97 stun. And maybe like 4 more for dwarves and 15 more for HGs, assuming the etwo double strength bonus.
I'm not even sure weapons with such high damdice exist. More likely, the difference between a halfsword and a maul is like 1d6 vs. 2d4.
An equivalent perfect backstab with a 1d6 weapon, without the etwo bonus, would yield (6x14.85)+3=92 damage. Now it's looking like you'd actually have to use a vastly better bludgeoning weapon for sap than the backstabber's stabbing weapon in order to match the damage. If you used a bludgeoner without abnormally high damage, the result would be crap.
If you had only 40 sap, the damage would have been pretty unimpressive. People like to say that the thug subguild lets you OHKO people, but the code doesn't show how this is supposed to be possible. If it is, the answer would have to be hidden somewhere outside of the skill's core damage calculations. I feel like if it was so viable, more people would be doing it. I admit I've literally never seen one PC sap another in like 13 years.
Maxed sap's multiplier caps at x9 while backstab gets x14.85. It gets no special strength bonuses or anything else that backstab doesn't get, just the flat strength bonus added at the end and then another "3 to (str_bonus or 4, whichever is greater)" if you etwo. The only defining difference (other than the multiplier) is the fact that you could use any bludgeoning weapon to sap with, including the biggest two-handed maul in the world, while backstab is limited to stabbing weapons which tends to mean daggers, shortswords and small spears. This is surely the reason sap's multiplier is much lower than backstab's. There are some very good bludgeoning weapons out there.
The stun damage bonus of bludgeoning weapons is specifically absent from the sap calculation, with a comment by Tiernan from 2002 about how he removed it because otherwise sap would do way too much damage. The code reveals no reason why sap should be more capable of one-hit knockouts, but there's always the possibility that the victim was at low stun from psionics. The same risk is there with backstab, though, as it does as much stun damage as health. It's common to backstab someone unconscious without bringing their health into the negative.
It's certainly unwise to backstab a skilled and armed warrior or ranger, but the target would have to have some balls to stick around and fight after you've chunked them down to poor/terrible condition. Add the fact that unlike sapping, backstabbing lets you use poisoned weapons, plus the excellent offense vs. humanoids and potential master piercing of an assassin makes it hard to trust that no subsequent swings will land. Assassins are shit defensively but top tier offensively. Suddenly the stab looks like the bigger threat.