Big edit 2020-09-05: Rewrite to accommodate a recent clarification of a long-standing GURPS rule (a recent clarification to me, anyway!):

In GURPS (and its offshoots like Dungeon Fantasy Roleplaying Game (DFRPG)), a target’s SM acts as a TH modifier for ranged and melee attacks against that target. That “melee” part is the clarification: it’s not made clear in the Basic Set (see this post in this forum thread), and many players (including me!) always assumed that the Basic Set didn’t make SM a TH mod in melee combat. But the official rule is that SM is a TH mod for melee attacks and ranged weapons alike.

So there it is. That revelation prompted an update to this article. In particular, the article’s core achievement of “unifying” GURPS‘ ranged and melee assumptions into a single procedure is lessened; its suggestion that the game should apply SM as a TH mod in both ranged and melee combat becomes moot, as the game already does so!

But the article’s further workings, particularly those leading to the suggestion that “relative SM” is the ideal way to handle SM as a TH mod in melee, still stand.

Intro: Grand Unification Theory

Imagine the following questions about combat in GURPS. (Fiddly, thoroughly non-pressing questions about special-case oddities, yes, but questions all the same.)

• SM acts as a modifier to TH in combat. The resulting near-impossibility of shooting at a mosquito (SM -15?) sounds fine, but that modifier applies to melee combat, too – so how can you possibly slap that bug?
• How can two tiny combatants – whether small critters or mini PCs – possibly punch each other when both face a near-impossible TH mod?
• Likewise, how could two tiny combatants ever hit each other with ranged attacks? Maybe they could do so by getting really close to each other – except:
• Getting up close to shoot a target naturally improves ranged attack TH – up to a mysterious threshold of two yards. Why does getting closer than two yards – such as shooting at a target from mere inches away – do nothing to aid TH?

All is answered: there exists a GURPS Unified Theory of Hitting Stuff (or GURPS Unified TH System; it’s GUTHS either way) that does the job. Okay, it’s not in the rules, and the name is my own goofy coinage. But this isn’t just some rules bit I’ve had posted on my site for ages, nor even much of a “house rule” at all. It’s a simple interpretation of existing GURPS rules that instantly sets all the above running smooth as a duck, 100% rules-as-written compatible.

The simple code

Enough of that. Here’s the GUTHS interpretation of how things get hit:

• 1) Whether GURPS knows it or not, its ranged and melee combat actually use all the same rules for TH! The core assumptions:
  • 1a) All TH, ranged and melee, uses the Size and Speed/Range table. Easy!
  • 1b) The Range column extends (well, should extend) its mods equally in both directions, offering bonuses for close range under two yards as well as penalties for range beyond two yards (just as it was nicely handled in 3e). Do away with the magic “combat range Planck length” of two yards. On your 4e table, replace those dreary “+0” Range entries with the appropriate bonuses!
  • 1c) From 1b, Range can logically be as close as circumstances allow, such as mere inches if you shoot a padlock to break it, or if you slap a bug on your arm. The appropriate big TH bonus applies, typically working miracles to offset a big TH penalty for small target size. But, when making regular attacks in combat against an active target (what with movement, uncertain distances and positioning, and need for maneuvering space), effective Range (melee or ranged) can only be as close as the attacker’s SM-based height/length (i.e., two yards for a human).
  • 1d) As has always been the case for both ranged and melee TH, target Speed may or may not be relevant: if the target is not in combat with the attacker, its Speed generally counts for Speed/Range purposes. But if the target is engaging the attacker, it can be considered to have no Speed for TH purposes.
  • 1e) This may be a wholly new addition to GURPS combat, but it’s a logical one: when a weapon with a large striking area strikes a small target, use the larger of the two as the effective target size. That covers wide flyswatters hitting tiny flies, or room-wide dragon-flame jets washing over PCs.
• 2) A couple of logical ranged-vs-melee considerations crop up, as follows:
  • 2a) While both ranged and melee attacks have an effective minimum Range (1c above), melee attacks also have a short maximum Range (typically a yard or two, longer for polearms and pikes).
  • 2b) From 2a, melee attacks face a cap on effective target size: any target size outside of a melee attack’s reach is meaningless as a target, and logically can’t aid TH. For simplicity, call that maximum effective target Size the attacker’s SM +4. (For a little more detail, that maximum might fall to +3 if your weapon is very short, i.e., Range C, or whatever the equivalent short range is for your size.)

Outcomes

From the above, the following outcomes fall into place:

Ranged combat

For ranged combat, applying all the above just boils down to “rules as written”. Follow standard 4e rules, using Speed (if relevant), Range, and Size (target SM) normally.

One important change occurs, though: do allow TH bonuses for Range closer than two yards, if that Range can be reasonably achieved. It can’t, for SM 0 humans shooting active targets (i.e., for most ranged combat situations); minimum range is the human’s height of two yards, which means no super-short-range TH bonus for claiming “I put my gun really close to the bad guy!” You can read 4e‘s stance of “no bonus for close Range” as reflecting that common situation.

But if a shooter can logically get closer to an unmoving target, do allow the appropriate close-range bonus. And even against an active and moving target, a small shooter’s effective range can go all the way down to his own height/length, not an arbitrary two yards!

This simple change makes many unusual situations – like those tiny gunmen having a close-range shootout, or a character shooting a lock from inches away – completely workable.

Melee combat

Melee, too, boils down to existing rules as written for typical situations. Target Speed is typically irrelevant in melee; ignore it, per normal GURPS rules.

Note, though, that under the above assumptions, each fighter strikes from an effective Range appropriate to his own SM, at a target Size of the foe’s SM. The net result of this is the same as using relative SM as a TH mod: the difference in SM becomes a bonus for the smaller combatant to hit the larger, and a penalty for the larger to hit the smaller. Nothing changes from normal GURPS rules for two human-like fighters, but odd situations (like two tiny fighters) now work!

(Note: this relative SM rule is offered as an option in the official GURPS 4e FAQ, thanks – at least in part – to Your Author lobbying for it from the GUTHS perspective. Though I’m sure many players have independently realized that it’s the sensible way to handle size in melee.)

A couple of little things:

• Don’t forget to place a maximum +4 on a smaller foe’s TH bonus for an attack vs a large target. (This is for melee only; 2b above doesn’t apply to ranged combat, and thus there’s no cap on your ranged attack TH bonus to hit a large target.) 
• Remember that you can hit small targets more easily by using the larger of weapon SM (based on its “striking area”) or target SM (from 1e above).

That’s it

With those simple interpretations in place, all the initial questions about size, speed, range and TH are answered. Dwarves vs Giants in melee, ants brawling with ants, creatures of every size shooting away at each other from any conceivable distance; everything is answered and everything works.

• A knife-fight between two pixies? It now works the same as a fight between two humans. (Except I’d pay more to see the pixie brawl.)
• Shooting a padlock from inches away? Sure; it’s a tiny target, but you can take a big bonus for tiny Range. Just watch for shrapnel…
• Need to capture that rare valuable butterfly, or deal with a teeny pixie pest? Use a weapon perfectly designed to overcome that vexing tiny SM by offering a big striking area: the butterfly net.
• Need to slap a mosquito? There’s likely zero need to game that with numbers and dice, but it’s nice that the engine can do so. The target is tiny, but you can use the hand’s SM, not the mosquito’s, as effective target size. Take a big Range bonus for having sneaked that slapping hand up close. Also don’t forget AoA (Determined) and Telegraphic Attack (especially if the bloodsucker is too busy feeding to even roll a defense). In the end, it’s an easy shot with GUTHS!

No existing rules have been mangled to enable the above effects; the rules have just been examined and made sense of. And ranged and melee TH are now unified in all aspects, which is cool, because… well, just because.

GUTHS. Spread it far and wide.