Tuesday, August 20, 2013

Using the Size Modifier Table for Rapid Fire in GURPS

GURPS has never had a very satisfactory set of rules for rapid fire. The original system in 2e failed to scale at all for weapons with high rates  of fire; the revised system in 2e and 3e required too many table look-ups, too many die rolls, and allowed for insanely high numbers of hits; the current version in 4e is fairly playable, but weapons with high rates of fire still get too few hits. There has to be a better solution.

Fortunately, there is. The Size Modifiers Rapid Fire system uses the same base mechanics as the 4e system, so it's a single roll to resolve all the shots fire, but it replaces the clunky and unmemorable Rate of Fire bonus table with the well-known and formulaic Size/Range modifier table. In doing so, it also creates a system that ensures that the same to hit roll will produce about the same percentage of shots on the target for weapons of the same recoil, no matter how many shots are fired.

The Size Modifier Rapid Fire system uses the same rules as Rapid Fire on B373 until you get to the RoF Bonus to Hit chart. Instead of looking the bonus up on that chart, look up the number of shots fired on the Size Modifier chart and use that bonus instead.
Example: Ted is firing an M-16 with RoF12 at full auto. 12 is more than 10 and less than 15, so Ted would get a +4 RoF bonus. If Ted were firing his semi-automatic shotgun with RoF 3x9, he would be firing a net 27 shots, for a +6 RoF bonus.
Calculate the Margin of Success for the attack normally, and then divide by the weapon's Recoil value to the get the Margin of Recoil. Subtract 1 from the Margin of Recoil and look up that bonus on the Size Modifier chart. The corresponding size measurement is the number of bullets that hit, up to the total number of bullets fired and to a minimum of 1.
Example: Firing an M-16 at RoF12, Ted makes the roll by 4. An M-16 has Rcl 2, so Ted's Margin of Recoil is 2. Subtracting 1 from 2, Ted gets a +1, which means 3 bullets hit. Had Ted made the roll by 0, his Margin of Recoil would have been 0, and he still would have hit with 1 bullet.
Example: Firing the shotgun, Ted makes the roll by 6. The shotgun has Rcl 1, so Ted's Margin of Recoil is 6. Subtracting 1 from 6, Ted gets a +5, which means 15 bullets hit. If Ted had made the roll by 8, he would have hit by all 27 bullets.
The Size Modifiers Rapid Fire System is elegant in three ways: it replaces an arbitrary table look-up with a look-up against a well-known and formulaic table; it scales up with the number of shots fired; and it ensures that firing more shots doesn't mean that a greater percentage of them hit. 

This table shows the number of shots that hit for a given RoF and recoil, assuming a 0 MoS attack without RoF bonuses:

Shots Hit (Margin of Recoil) by Recoil
Shots Fired Bonus 1 2 3 4
1 0 1 (0) 1 (0) 1 (0) 1 (0)
2 0 1 (0) 1 (0) 1 (0) 1 (0)
3 1 2 (1) 1 (0) 1 (0) 1 (0)
5 2 3 (2) 2 (1) 1 (0) 1 (0)
7 3 5 (3) 2 (1) 2 (1) 1 (0)
10 4 7 (4) 3 (2) 2 (1) 2 (1)
15 5 10 (5) 3 (2) 2 (1) 2 (1)
20 6 15 (6) 5 (3) 3 (2) 2 (1)
30 7 20 (7) 5 (3) 3 (2) 2 (1)
50 8 30 (8) 7 (4) 3 (2) 3 (2)
70 9 50 (9) 7 (4) 5 (3) 3 (2)
100 10 70 (10) 10 (5) 5 (3) 3 (2)
150 11 100 (11) 10 (5) 5 (3) 3 (2)
200 12 150 (12) 15 (6) 7 (4) 5 (3)
300 13 200 (13) 15 (6) 7 (4) 5 (3)
500 14 300 (14) 20 (7) 7 (4) 5 (3)

Given that the Margin of Recoil values are roughly equal to the number of shots that would hit under the normal 4e rules, I think it's clear that the Size Modifier Rapid Fire rules produce a better scaling of shots hits to total shots fired, at no additional cost in complexity.

Inspired by this thread on the SJ Games forums. http://forums.sjgames.com/showthread.php?p=1631294

Edit: What About Dodging?

I forgot to address rules for dodging in the original draft of this article, but they're easy enough to figure out. A successful Dodge defense reduces the Margin of Recoil by 1 plus the MoS on the Dodge roll.
Example: Ted fires an RoF 3x9, Rcl 1 shotgun at an alien and makes his roll by 6, hitting with 15 shots. The Sectoid dodges and drops, succeeding on its defense roll by 3. Ted's Margin of Recoil is reduced from 6 to 3, so only 7 shots hit the Grey.
Example: Ted's partner Allen opens up with an RoF12, Rcl 2 assault rifle on another Sectoid, making the shot by 4 for a Margin of Recoil of 2 and 5 shots hitting. This Sectoid also drops and dodges, succeeding by 3. All of Allen's shots miss.


  1. It occurred to me after posting that while it isn't entirely insane to dodge the 6 ppellts of buckshot that hit after a decent roll with a 3x9 shotgun, dodging the 15 pellets that this system would produce is much more difficult.

    Which brings up a good question that I can't decide how to answer: should dodges against RoF be on a 1:1 basis like they are now, or should dodges be on the Size Modifier table too, so to dodge 15 pellets you only need to succeed by 6 or so?

    1. Seems like a logical course of action. If we were talking about a system with a bit more abstraction - longer than one-second turns, for example - it might be appropriate, especially for walking fire onto a target, for the dodge to be more difficult, but given the nearly instantaneous nature of these attacks in GURPS, what's good for the good is good for the gander.

    2. What would be the game mechanical impac of dodge decreasing effective SM or flat out margin of success? Or increasing Rcl?

  2. I like it. Especially for high RoF situations like in spaceship combat, where the normal rules completely break.
    I think that dodge should decrease the MoS before calculating the number of hits, like Douglas Cole suggested. Otherwise you get lots of hits and no way to dodge them.

    1. Having thought it over, you're right.

      It's a little weird that it's easier to dodge 100 shots when there are 1000 headed toward you, but it makes sense when you think about the abstraction more. A rapid fire attack attack is basically trying to center a cone of attacks on the target, and a dodge represents moving toward the edge of the cone where the density of attacks is much lower. With that view, reducing the number of hits from 20 to 15 to 10 by succeeding in a Dodge attempt by 0, 1, or 2 makes more sense, since that's basically moving from the center of the cone toward the edges.

      I should update the post.

    2. This comment has been removed by the author.

  3. Coming back to this after a long absence, you could also deal with size modifier of the target directly by adding to Margin of Recoil. If you hit with 7 shots on an SM +0 critter, then you'd hit with 15 on SM +2, because it covers twice as much of the cone.


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