Bullet flight time conventions

[Joseph Paul]

A couple of current threads about TL8-9 weapons and others made me curious.

What are the current conventions and assumptions about bullet flight time in GURPS? What do you use?

Just what does Max Range represent in GURPS? The furthest reach of a normal firing of a round parallel to the ground? Or the deliberate attempt to get the most range out of it by firing in an arc?

RAW for firearms is that Max range can be reached in one turn of one second's duration. Frankly, while playable, it implies some very implausible muzzle velocities. And that gets worse with ETC and ETK options. What were Mr. Pulver's ideas on where muzzle velocity would go in the future with UT (and range and damage) and why?

Flight time issue - with the advent of Guided and Homing munitions in UT flight time became an issue. No longer limited to just specific missile weapons in Basic any firearm can add these options to it's ammo.

On the one hand that still creates some highly improbable muzzle velocities. The 15mm AM rifle has a 1/2D range of 2000 yards that would be a 6000 FPS muzzle velocity. Modern cartridges are topping out at 4000 FPS and they have severe limits on their actual range - some of those have a poor ballistic coefficient that limits the range to a few hundred yards.

On the other it is not possible to explain why a round that could do 9000 yards in a second now takes 4.5 seconds to cover that distance just because there is a homing package on it that adds no weight and doesn't change the damage. So that is some cognitive dissonance.

Tactical Shooting (a book I don't have so I may be mistaken about the details) introduces a bullet flight time standard of 250 yards per turn for handguns and 600 yards per turn for rifles to use in figuring bullet flight time. 1/2D Range has no effect on rounds that are not Guided or Homing, use the 250/600 divisor to find how many turns it takes your round to arrive. Does TS give any changes for missiles etc that would use 1/2D Range as the divisor to find flight time?

Guided and Homing - for missiles with switchblade fins and active controls it is pretty easy to see that they have some good capability to make some pretty hardy deviations to follow a target. However a number of firearms opting to use G&H ammo are not set up for that kind of control surface. At best they will have some small active system producing thrust for maneuver through the time of the flight that can be used or with TL9+ materials active surfaces that induce drag to make changes. Either way the degree to which the round can change direction is limited and the high velocity of some 1/2D Ranges should shrink that capability further. Should we be thinking in terms of a circle of possible correction that shrinks each turn until the round hits or runs out of range? That would also give the target opportunity to break the contact - the sensor may still detect the target but it can't produce enough correction to actually hit.

I see the possibility of defining muzzle velocities for some of these weapons and playing with variations. I am wondering if there is a muzzle velocity advantage that I can give to EM firearms that will keep them ahead of conventional firearms and even with ETC/ETK?

Or is it more reasonable to limit the advantages of ETC/ETK to one or more of damage, increased 1/2D range and not much extension of Max range citing problems with ballistic coefficients at those speeds/mass/length of bullet? Or reducing the multipliers for ETC/ETK?

[AlexanderHowl]

In the case of projectiles, I generally assume that 1/2D range represents the velocity of a round in yards/second. Max range then represents the maximum range for a flat trajectory based on air resistance, gravity, etc. (ballistic trajectories have a greater range dependant on the angle). When attacking beyond 1/2D range, I generally give targets a +1 to Dodge per 1/2D interval beyond 1/2D range (rounded up) if they are aware, mobile, and know roughly where the attack is coming from, as they have time to get out of the way.

In the case of direct energy weapons, I generally rule that 1/2D range just represents optimal range and max range represents effective range. Beyond max range, a directed energy weapon is generally harmless, though very powerful weapons might be dangerous beyond max range, divide by 10 for every max range interval. For example, a laser cannon that deals 100d damage at 100 miles max range would still deal 10d damage at 200 miles and 1d at 300 miles.

[Ulzgoroth]

Flight time: Tactical Shooting gives a couple numbers to use. I don't love those numbers, but they're there. You really should have different numbers for some weapons, particularly (mostly UT) high-velocity weapons and low-velocity weapons like grenade launchers, but other than muzzle velocity/half muzzle velocity much past 1/2d/maybe about a third muzzle velocity near max range, I don't have much to go on. And of course for UT weapons you usually have no way to determine muzzle velocity. UT has issues.

Tactical Shooting isn't changing anything with that. Prior to it there just weren't any rules about flight time for non-guided rounds except maybe artillery.

Also, 1/2d I'm given to understand is Not Done Right in a significant number of canonical sources and I'm not sure whether that's all of them or only a subset.

Max range is a maximum range at which you can use the weapon. It usually factors in both projectile ballistics and what elevation is usable. I have no information about what book authors actually do with it in many cases, but that's what it tells us. It absolutely isn't maximum range for a shot 'parallel to the ground', that's not how projectile weapons are used in any context where maximum range is a consideration.

The rule from UT for flight time for non-guided projectiles converted to guidance is nonsensical and lazy, and I would never ever follow it as written.

Guidance maneuver limits...yes, they is a thing (even for missiles with maneuvering surfaces for that matter). But they only really factor in against highly maneuverable targets, which GURPS doesn't really model all that much. Ground vehicles and infantry generally aren't going to be able to do anything that could force a miss in that way. But of course aircraft can and spacecraft might.

Originally Posted by AlexanderHowl

In the case of projectiles, I generally assume that 1/2D range represents the velocity of a round in yards/second.

That's obviously, egregiously wrong. GURPS stat-lines using the same spot in the format for the flight speed of guided weapons and the 1/2d range for other weapons does not mean those numbers are related.

[RyanW]

Originally Posted by Joseph Paul

Just what does Max Range represent in GURPS? The furthest reach of a normal firing of a round parallel to the ground? Or the deliberate attempt to get the most range out of it by firing in an arc?

Any but the closest shots require some arc. If fired parallel to the ground from shoulder height, the bullet will strike the ground in about half a second.

[NineDaysDead]

I think there was a pyramid article about this?

[Joseph Paul]

Did bullet flight time, whether for firearms or artillery, get covered in 3e?

[RyanW]

Originally Posted by Joseph Paul

Did bullet flight time, whether for firearms or artillery, get covered in 3e?

I think HT 3e included some rough numbers, in the context of calculating the arrival time of artillery.

[Fred Brackin]

Originally Posted by Ulzgoroth

The rule from UT for flight time for non-guided projectiles converted to guidance is nonsensical and lazy, and I would never ever follow it as written.

d.

What rule would this be? Can you cite a page number? For the 64mm and 100mm missiles the two distances shown are indeed velocity per Turn and max range but I do not believe anything is said about what happens when you put guidane on conventional bullets. The passage about doing that could be expunded with little loss.

To the OP I can tell you that the velocity increase for ETC is 50%, same as for damage. Otherwise the velocities for plain caseless rounds should proably be not very different from those of contemporary guns.

Finally, What do _I_ do about this in my games? Nothing. It's never come up.

[Rupert]

Originally Posted by AlexanderHowl

In the case of projectiles, I generally assume that 1/2D range represents the velocity of a round in yards/second.

That really doesn't work as a general rule.

It works for the 7.62x39mm, as it happens to take just about a second to reach its 500 yard 1/2D range.

However, 5.56x45mm has a 1/2D range of 500 or 800 yards, depending on the load used, but the SS109 bullet (800 yards 1/2D) takes about 1s to get to ~650 yards.

A 7.62x51mm ball round gets 770 yards in the first second of flight. Most weapons using 7.62x51mm ammo are given 1000 yards for their 1/2D range.

It's even worse for pistols. A 9x19mm pistol has a 1/2D of about 160 yards, but such bullets travel over 300 yards in their first second of flight.

It's also entirely possible for light, fast bullets that should have lower 1/2D range to get further in their first second of flight than slow, heavy bullets with better ballistic characteristics and thus longer 1/2D ranges.

If it mattered, I'd just go with the 250 yards/s for pistols and low velocity rifles, and 600 yards/s for high velocity rifles, on the basis that at ranges where it matters the bullets will be moving slowly, for all that they started out moving smartly. More precision seems pointless given the variability of real-world small arms ballistics.

[Rupert]

Originally Posted by Ulzgoroth

Also, 1/2d I'm given to understand is Not Done Right in a significant number of canonical sources and I'm not sure whether that's all of them or only a subset.

It depends on what you mean by 'not done right'.

If 1/2D range is looked at as being when the projectile has slowed to the point where it does half the damage it does at the muzzle, then many of the ranges will be wrong. 1/2 damage implies 1/4 velocity, which can be a surprisingly long way.

Also, strictly speaking the 1/2 range won't change when firing a given round in different weapons - initial velocity does not determine how far the bullet needs to travel before dropping to 25% of muzzle velocity, merely how fast and thus how much damage it did to start with. All bullets with the same degree of streamlining and same sectional density will have the same 1/2D range. Also, this mean that each halving of damage occurs after the same distance, so 1/4D range is double 1/2 range, 1/8D is three times, and so on.

BTW changes in atmospheric density will change 1/2D range, so at extreme altitudes and on other worlds it will be more (low pressure) or less (high pressure).

Max range changes with air pressure and with gravity, and unlike 1/2 range does depend on muzzle velocity (as well as streamlining and cross sectional density). Thus 1/2D range shouldn't change between weapons firing the same round, even when they have radically different barrel lengths (e.g. firing a pistol round from a carbine), but max range should.

I think that in practice 1/2 ranges have tended to reflect 'effective range' from literature, and a bit of the 'intuitive' 'longer barrels make things go further' and 'big bullets go further'. For example, in HT (which is otherwise pretty good), the G3 has a shorter 1/2D range than other 7.62x51mm battle rifles, and the Model 70 Winchester firing .458 Winchester is given a very long 1/2D range as is the .600 Nitro Express, even though they both fire quite poorly streamlined bullets - that's why long range large game shooting tended to use .375 H&H and similar rounds.

Re-doing all of this to make everything dead-consistent is pretty much pointless, and a huge amount of work for little gain.

Max range is a maximum range at which you can use the weapon. It usually factors in both projectile ballistics and what elevation is usable. I have no information about what book authors actually do with it in many cases, but that's what it tells us. It absolutely isn't maximum range for a shot 'parallel to the ground', that's not how projectile weapons are used in any context where maximum range is a consideration.

Attempt to find a maximum range in a good source is what they do, I think. Failing that, make an educated guess based on what similar weapons manage.

For UT weapons any 1/2D is going to be an approximation, and max range an approximation off that approximation. There's no sensible way around this.