Radical Alternatives: How SHOULD Size and Speed/Range Affect Chance to Hit?

[vicky_molokh]

Greetings, all!

I'm doing some musings about the Size and Speed/Range Table. Currently, the SSRT scales as a logarithm of linear distance, and each point of penalty affects the chance to hit in accordance to the 3d6 probability table. But if one were to make a radical rewrite of the relationship between range and chance to hit, what should it be? Note that I'm talking about the chance/probability more directly, not necessarily constraining it to the current 3d6 curve.

Asking because as far as I understand, for a given cone of likely directions a projectile can go, every doubling of distance quadruples the the area of the beaten zone, thus reducing 'bullet density' four-fold.

Similarly, currently speed is being added to distance directly, but is there perhaps a more elegant way to make target (and perhaps shooter) speed affect hit ratios?

Note: normally I criticise radical rewrites that result in other bits of the system breaking down hard, but this is more of a hypothetical question with the hopes of seeing how things would look if designed without worry about legacy support and with an eye towards verisimilitude and general universality.

Thanks in advance!

[Anthony]

Originally Posted by vicky_molokh

But if one were to make a radical rewrite of the relationship between range and chance to hit, what should it be?

Realistically, it's likely that aiming at an unmoving target follows some variant on Fitt's law, and aiming at a moving target is equivalent to the steering law. What these boil down to is that the time to aim in the right general direction is proportional to the logarithm of (distance gun has to move to aim at target)/(size of target), and whether you can successfully keep your gun pointed at the target is dependent on the ratio of the target's positional uncertainty to the target's speed.

This has the somewhat surprising effect that hit probability is not correlated to range -- for a target that pops into view for a brief period it's dependent on the size of the area you're watching vs the size of the object, for a target moving evasively it just depends on speed vs area, and in both cases the actual effect is on how long it takes to line up the shot, not the actual accuracy you can achieve. Range comes back into play in a couple ways:

• Mechanical Accuracy: there's a limit (determined by skill and stabilization) to how accurately you can aim a gun, and also a limit to how accurate the gun itself can be. Once you drop below that limit, hit probability will drop off quite fast.
• Bullet Drop: you have to aim above the target at long ranges. The amount by which you miss is equal to 0.5 * (range error) * (range) * (gravity) / (bullet speed^2).
• Leading the target: you have to aim ahead of the target at long ranges. The amount by which you miss is equal to (range error) * (target lateral velocity) / (bullet speed).
• Predicting the target: an erratically moving target might just move out of the way; this is uncorrectable error best implemented as an active defense.

[Eukie]

Very little data has been published on actual aim time/accuracy tradeoffs in ranged weapon use, though what I found seemed to suggest that the standard deviation in accuracy was proportional to time spent aiming to the ^-x power, where x is a number 1 < x < 2, such as 1.22 - which does map to some empirical power law results for Fitts's Law.

Originally Posted by Anthony

Range comes back into play in a couple ways:

• Bullet Drop: you have to aim above the target at long ranges. The amount by which you miss is equal to 0.5 * (range error) * (range) * (gravity) / (bullet speed^2).
• Leading the target: you have to aim ahead of the target at long ranges. The amount by which you miss is equal to (range error) * (target lateral velocity) / (bullet speed).

This is broadly true of ballistic weapons in a vacuum, but the environment where players are most likely to use ranged weapons is in an atmosphere. Here the interactions of the projectile with the atmosphere becomes quite significant.

Additionally, when leading, the error in target lateral velocity estimation is also significant. (British studies on tanks in WWII suggested something like an average estimation error of 30%)

[Anthony]

Note that Fitt's law was originally designed for analysis of the difficulty of using a control panel, and is also applicable to computer UI design, but while that probably implies that it's also applicable to hitscan weapons in FPS games, deciding it also applies to an actual physical gun is at best an unproven hypothesis.

[vicky_molokh]

Hmm. Talking to a shooting enthusiast resulted in some support of the quadratic hypothesis: he said that typically, someone with a 50% hit rate at 100m would have a 12½% rate at 200m against the same target under similar conditions. But in an RPG context, that seems to lead to hit chances quickly approaching 100% and 0% outside a relatively narrow range band, and I'm not sure how to feel about that.

As for Fitt's law, that seems to not be very applicable, since usually a turn length is defined before rolling (and is in GURPS and most systems a constant value).

[Gnomasz]

Originally Posted by vicky_molokh

As for Fitt's law, that seems to not be very applicable, since usually a turn length is defined before rolling (and is in GURPS and most systems a constant value).

Turn length, yes. But aiming time could be varied depending on roll. If you roll well, you shoot and hit. If you roll not-so well, but decent, you continue aiming. If you roll really poorly, you shoot and miss.

[johndallman]

Originally Posted by vicky_molokh

Hmm. Talking to a shooting enthusiast resulted in some support of the quadratic hypothesis: he said that typically, someone with a 50% hit rate at 100m would have a 12½% rate at 200m against the same target under similar conditions.

Yup, that's the inverse square law. Applying that to GURPS requires deciding how to convert that to a skill penalty.

The GURPS 3d6 mechanic's bell curve means that you pretty well have to start at the centre of the curve, and say that doubling range takes target number 10 (50% chance of a hit) down to target number 8 (25.6%). So far, so good, but not very granular.

The speed/range table has x10 distance as -6 to skill. By inverse square law, x10 distance should be 1% of the chance to hit, taking 50% down to 0.5%. That's the chance we have for a skill of 3, implying that x10 distance should be -7 to hit, so GURPS RAW is actually a bit friendlier than reality.

[Anthony]

Originally Posted by vicky_molokh

Hmm. Talking to a shooting enthusiast resulted in some support of the quadratic hypothesis: he said that typically, someone with a 50% hit rate at 100m would have a 12½% rate at 200m against the same target under similar conditions. But in an RPG context, that seems to lead to hit chances quickly approaching 100% and 0% outside a relatively narrow range band, and I'm not sure how to feel about that.

For people approaching their accuracy limits against unmoving targets, hit chance really does vary like that. The thing is, the reasons for missing at short ranges are not accuracy related; they're related to time to aim vs target motion, rushing the shot, and bad visibility.

[Anthony]

Originally Posted by vicky_molokh

As for Fitt's law, that seems to not be very applicable, since usually a turn length is defined before rolling (and is in GURPS and most systems a constant value).

The physical argument for Fitt's law is basically that you move your point of aim (or hand, or whatever) from its current position to where it will actually hit by making a series of corrections, where each correction has an error that's a fraction of the size of the correction. The complication for real shooting is that there is additional random error (which depends on stance, bracing, etc), which will exceed the fractional error at a certain point, capping your accuracy. However, that error is pretty small -- basically, look at the minimum group size that can be achieved at a given range for a given weapon and grip/stance, and compare to the size of a target. This is generally good enough for automatic hit on a torso-sized with an unbraced handgun at ten meters, and a combination of proper bracing and a long gun can push that out to hundreds.

Real-world hit chance for pistol fights at under ten meters is about 20%.

[Pursuivant]

One big issue that GURPS misses when dealing with Range/Speed calculations is deflection shooting.

It's generally a trivial problem when aiming a relatively close and slow-moving targets, like humans maneuvering in a firefight, but it becomes a real hassle when shooting at more distant and faster-moving vehicular targets.

As an example, it was estimated that aerial gunners during World War II put less than 1% of their bullets on target in combat when shooting at enemy fighters (say SM 3 to 5, traveling at relative speeds of 100 to 1,500 yards/meters per second, at ranges of 100 to 1,000 yards/meters). Just 2% accuracy was expected for graduates of aerial gunnery schools, based on shooting at target drogues pulled by relatively low-speed aircraft flying predictable patterns while firing from a relatively low-speed airplane.

Even shooting at a target coming straight at you (0 degrees deflection) is a potential problem if you don't know the range and there's significant bullet drop.

Arguably, Speed penalties should be increased if taking the shot requires you to quickly traverse your gun while also estimating range and "angle off" on the fly.

Long range shots should be further penalized by speed modifiers if there's any significant projectile travel time unless you have some means to accurately predict range, target motion, flight time, and projectile drop.

Another issue is Inherent Accuracy, especially when dealing with lower TL weapons at longer ranges. For example, a TL5 smoothbore musket is going to have a target grouping which exceeds the area covered by a man-sized target at much more than 50 yards/ meters of range. (One military writer of the time wrote that you had as much chance of hitting the man on the moon with a musket shot as hitting a man standing at 100 paces.)

GURPS 3E made a clumsy attempt to model this by ruling that Acc bonuses didn't apply beyond 1/2D range, but a simpler rule might be to rule that your maximum modified skill with any ranged weapon not specifically designed for accurate long-ranged shooting can't exceed the weapon's TL + 10.