If you're going to make your PCs immune to bullets, I'm not sure why you would even bother to have guns in the setting. Their inclusion would only increase the PCs' power.
Hence why I'm revising and toning down the rules a bit.
That said, some of the party are wearing Kinetic Armour anyway, so they'll laugh off any bullets that aren't flechette or HEAP anyway - the latter of which is assumed to be pretty much standard armament for a modern military force that would be expected to engage with them. (A several thousand year-old robot of not that far past modern technology levels, not so much!)
I would pick #3, which is the by-the-book answer.
I'm leaning more that way myself at the moment.
If you wanted an intermediate interpretation, you could give a penalty for firearm attacks on the basis that the attacker needs to aim for areas that are more solid rather than shooting through the gaps. Skull, sternum, pelvis, shoulder, etc. Maybe -10.
I would probably agree with Jdale on this point but I would make the penalty to hit more like -25 or more. Having competed in IDPA shooting events. If my biggest target is the head and it is a moving target, my penalty to hit cannot be overstated. Most action shooting and IDPA scenarios are against stationary targets and not a moving skeleton with a very limited surface area to hit.
But if you do that, you basically have to make the same argument for every attack by the same logic. Which is basically giving them an arbitrary DB boost.
(And I REALLY do not want to give the PCs any extra DB, we switched parties TO the liches working for one of the top most advanced major galactic powers precisely
because the regular adventurer's DBs were getting obscene!)
If you did that, you 'd have to drop the reduction of critical severity, because that's basically the same thinking, just from a different angle. The point about having the crits reduced in severity was that it made them harder to damage. I think fiddling with the crits is a more elegant (and less potentially game-breaking) way of getting that result than just giving the PCs more DB.
(Higher DB means less of anything connects. Crit reduction means the PCs still take some concussion hit damage from the attack table, which, even on just a purely psychological level, keeps the players a bit more honest and doesn't give them the same sense of invulnerability.)
If you scored a hit against the skull, a modern bullet would completely obliterate the skull.
Not any more or less you would hitting a regular human with the same bullet, though, really.
Even if there is a disparity, you can explain it away as being with the reasonable argument that an RM Lich/Skeleton bone structure is at least if not slightly more damage resistant than one encased in flesh, considering Undead in RM2 actually regenerate (most) damage. (And is an argument that is accurate given the universe fluff in question.)
A very interesting fantasy game you are running.
More scifi than fantasy, I think (given fairly broad definitions). It's principally a (soft-ish) sci-fi setting, it's just one where magic exists and thus some races/groups have access to it more than others. The group in question are members of the Aotrs (Army Of The Red Spear), which is major power comprised principally (militarily) of spirit-bound Liches under a Grand Admiral Thrawn-y sort of Lichemaster who have basically optimised their magic and technology with their Undead traits to reach the position where they are a small but extremely powerful galactic power.
(The really strange thing; the fact this is an evil party of Liches is more-or-less coincidental to the thrust of this party, which essentially sort of Stargate SG-1-y explore-y. It ended up being about the Aotrs when I realised they were the power I actually had the most detail on (down to weapons and everything) and even the (extensive amount of) ground vehicles for (for playing wargames at 144th and 72nd scale). Had I chosen pretty much anyone else, I would have had EVEN MORE work to do to lay the ground work! And considering the amount of time I've had to sink in (even liberally borrowing from Stargate SG-1 D20) - I started in the back end of
May - it was definitely the right choice, if a little unorthodox...!)
a 71A slash causes tendon damage at -30. I 1/2 the penalty and assume a light break at -15.
I think I had included that as well, at once point, but took it out on the last revision, as reduce critical AND immune to not-bone damage on several critical tables AND halve penalties seemed to be getting ridiculous.
I suppose one might argue you could say:
4) Immune to stun and bleeding. Reduce all criticals by one level of severity. Reduce all Cold criticals by four levels of severity. Unaffected regular (non-ballistic) Puncture criticals, reduce Ballistic Puncture, Ballistic Hollowpoint, Ballistic Armour-Piercing, Shrapnel and Ballistic Shrapnel by two levels of severity.