GURPS Overhaul - Rate of Fire

Varyon · 09-05-2013, 04:05 PM

I've debated posting this, as I believe Anthony's houserule in the RoF thread is a good deal more gameable, but ultimately decided not to have all my work go to waste. To design this system, I made (in Photoshop) circles of various SM's (between -10 and 1) and overlapped them. I assumed that hitting an SM "on the number" (MoS 0) meant centering the "hit sphere" exactly between its perimeter and the perimeter of the next smallest SM. I also made the assumption that distribution of bullets within the hit sphere was completely random, meaning how close the targeted area was to the center of the sphere made no difference. I believe this is indeed the case with shotguns, but may not be true of other weapons. Please note that if you use this variant, you should probably use a semi-cumulative wounding system, rather than GURPS default, as otherwise this will make high RoF weapons far too lethal.

This system replaces the existing RoF rules. First off, high RoF does not (directly) give a bonus to the chance to hit. Also, weapons no longer have a Rcl statistic, rather they have Spread, which is a negative number. The more negative the number, the less spread the weapon suffers when firing at maximum RoF. Unchoked shotguns firing shot have Spread -1 (one inch spread per yard of range, or 1/36 radians), while a mounted machine gun may have Spread -12 (one inch spread per ~28 yards of range, or 1/1000 radians).

Spread represents the maximum Size + Range penalty the weapon can suffer before it becomes possible to hit with a fraction of the shots fired (rather than all-or-nothing) [i]assuming the weapon stays on-target[i] (see later). Speed does not factor into this calculation (but note you still are penalized as normal for it). For an unchoked shotgun, this would be when firing at an SM+0 target at 3 yards, while for a mounted machine gun you're looking at shooting an SM +0 target at 200 yards. For high-spread weapons, it may be more appropriate to use the distance from the muzzle to the target (in the case of a typical shotgun, I think this adds around 1 yard to this distance), rather than from the user to the target.

At penalties beyond Spread, use the following table to determine what fraction of the rounds fired actually hits. I assume here that spread follows a completely random pattern within the sphere - that is, a bullet is as likely to hit a given area as any other area the same size (provided both areas are fully within the spread "sphere"), regardless of how close to the center of the spread these areas are.

Code:

	-3	-2	-1	0	1	2	3	4	5	6	7
0	0	0	0	1	1	1	1	1	1	1	1
-1	0	0	0	0.95	1	1	1	1	1	1	1
-2	0	0	0.05	0.95	1	1	1	1	1	1	1
-3	0	0	0.05	0.8	1	1	1	1	1	1	1
-4	0	0	0.15	0.65	1	1	1	1	1	1	1
-5	0	0	0.25	0.65	0.95	1	1	1	1	1	1
-6	0	0	0.2	0.45	0.65	0.75	0.8	0.85	0.9	0.95	1
-7	0	0.05	0.25	0.35	0.45	0.45	0.45	0.45	0.45	0.45	0.45
-8	0	0.15	0.25	0.25	0.25	0.25	0.25	0.25	0.25	0.25	0.25
-9	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1

The numbers across the top are Margin of Success (note MoS -1 is the same as MoF 1), while the numbers down the left side are for the Range+Size penalty relative to Spread (so shooting at SM+1 target at 30 yards is a -6 total penalty, so if using an unchoked shotgun - Spread -1 - you would use the line corresponding to -5). For penalites beyond Spread-9, each additional -1 increases the MoF range by 1 and halves the number of hits (so at Spread-10 you can have MoF 4 and still hit, but will only hit with 5% of the shots; at Spread-15, you can go down to MoF 9 but will only hit with 0.1563%). For ease of use, don't assess any range penalty beyond Spread-9 (you're essentially getting +1 per -1) and follow a 1-2-5-10 pattern (so our Spread-15 example would hit instead with 0.1%).

Of course, GURPS RoF is given in shots per second, and the initial roll just tells you where you start - keeping on-target for an entire second isn't exactly easy. Make a Per-based weapon or Observation roll at -10, plus any penalty due to speed (but not range). Sighted shooting applies the weapon's Acc or +4 (whichever is larger) to the roll. Having all rounds with clearly-visible impacts (explosive rounds, or incendiary rounds in low light) or using a tracer mix grants +2, while having both or using all tracers grants +4 (using all explosive tracer rounds has no additional effect). Add you MoS (or MoF) to your initial MoS to determine how close you are to the target at the end. The actual percentage of rounds that will hit is going to be an average of all points from start to finish. For example, let's say you're firing a mounted machine gun (Spread -12) at an SM+1 vehicle that is 500 yards away and traveling at Move 30. 500 yards is -14, so with SM+1 we're at -1 from Spread. Let's say we roll well enough for MoS 3. Now we need to stay on-target with our Per-based roll, working against a penalty of -17 (-10 base, -7 speed). Let's say we end up with MoF 5. We add this to our previous result, for a total MoF 2. Now we need to average all our hits together to determine the actual hit pecentage. MoS 3 is 100%, as are MoS 2 and 1. MoS 0 is 95%, and MoF 1 and 2 are each 0%. 3.95/5=.79, for 79% hits. Were we firing at RoF 30, we'd be looking at 23 or 24 hits (average is 23.7).

The big problems with this system are the need for multiple rolls, the need to calculate average number of hits, and, most importantly, a lack of Spread values for weapons - seriously, shotguns I got from Tactical Shooting, mounted machine guns from a mention in the other thread, and I have no clue about the spreads of other weapons (other than that they probably have Spread between these two).

Next up, an option to randomize number of hits!

09-05-2013, 04:05 PM	#1
Varyon Join Date: Jun 2013	GURPS Overhaul - Rate of Fire I've debated posting this, as I believe Anthony's houserule in the RoF thread is a good deal more gameable, but ultimately decided not to have all my work go to waste. To design this system, I made (in Photoshop) circles of various SM's (between -10 and 1) and overlapped them. I assumed that hitting an SM "on the number" (MoS 0) meant centering the "hit sphere" exactly between its perimeter and the perimeter of the next smallest SM. I also made the assumption that distribution of bullets within the hit sphere was completely random, meaning how close the targeted area was to the center of the sphere made no difference. I believe this is indeed the case with shotguns, but may not be true of other weapons. Please note that if you use this variant, you should probably use a semi-cumulative wounding system, rather than GURPS default, as otherwise this will make high RoF weapons far too lethal. This system replaces the existing RoF rules. First off, high RoF does not (directly) give a bonus to the chance to hit. Also, weapons no longer have a Rcl statistic, rather they have Spread, which is a negative number. The more negative the number, the less spread the weapon suffers when firing at maximum RoF. Unchoked shotguns firing shot have Spread -1 (one inch spread per yard of range, or 1/36 radians), while a mounted machine gun may have Spread -12 (one inch spread per ~28 yards of range, or 1/1000 radians). Spread represents the maximum Size + Range penalty the weapon can suffer before it becomes possible to hit with a fraction of the shots fired (rather than all-or-nothing) [i]assuming the weapon stays on-target[i] (see later). Speed does not factor into this calculation (but note you still are penalized as normal for it). For an unchoked shotgun, this would be when firing at an SM+0 target at 3 yards, while for a mounted machine gun you're looking at shooting an SM +0 target at 200 yards. For high-spread weapons, it may be more appropriate to use the distance from the muzzle to the target (in the case of a typical shotgun, I think this adds around 1 yard to this distance), rather than from the user to the target. At penalties beyond Spread, use the following table to determine what fraction of the rounds fired actually hits. I assume here that spread follows a completely random pattern within the sphere - that is, a bullet is as likely to hit a given area as any other area the same size (provided both areas are fully within the spread "sphere"), regardless of how close to the center of the spread these areas are. Code: -3 -2 -1 0 1 2 3 4 5 6 7 0 0 0 0 1 1 1 1 1 1 1 1 -1 0 0 0 0.95 1 1 1 1 1 1 1 -2 0 0 0.05 0.95 1 1 1 1 1 1 1 -3 0 0 0.05 0.8 1 1 1 1 1 1 1 -4 0 0 0.15 0.65 1 1 1 1 1 1 1 -5 0 0 0.25 0.65 0.95 1 1 1 1 1 1 -6 0 0 0.2 0.45 0.65 0.75 0.8 0.85 0.9 0.95 1 -7 0 0.05 0.25 0.35 0.45 0.45 0.45 0.45 0.45 0.45 0.45 -8 0 0.15 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 -9 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 The numbers across the top are Margin of Success (note MoS -1 is the same as MoF 1), while the numbers down the left side are for the Range+Size penalty relative to Spread (so shooting at SM+1 target at 30 yards is a -6 total penalty, so if using an unchoked shotgun - Spread -1 - you would use the line corresponding to -5). For penalites beyond Spread-9, each additional -1 increases the MoF range by 1 and halves the number of hits (so at Spread-10 you can have MoF 4 and still hit, but will only hit with 5% of the shots; at Spread-15, you can go down to MoF 9 but will only hit with 0.1563%). For ease of use, don't assess any range penalty beyond Spread-9 (you're essentially getting +1 per -1) and follow a 1-2-5-10 pattern (so our Spread-15 example would hit instead with 0.1%). Of course, GURPS RoF is given in shots per second, and the initial roll just tells you where you start - keeping on-target for an entire second isn't exactly easy. Make a Per-based weapon or Observation roll at -10, plus any penalty due to speed (but not range). Sighted shooting applies the weapon's Acc or +4 (whichever is larger) to the roll. Having all rounds with clearly-visible impacts (explosive rounds, or incendiary rounds in low light) or using a tracer mix grants +2, while having both or using all tracers grants +4 (using all explosive tracer rounds has no additional effect). Add you MoS (or MoF) to your initial MoS to determine how close you are to the target at the end. The actual percentage of rounds that will hit is going to be an average of all points from start to finish. For example, let's say you're firing a mounted machine gun (Spread -12) at an SM+1 vehicle that is 500 yards away and traveling at Move 30. 500 yards is -14, so with SM+1 we're at -1 from Spread. Let's say we roll well enough for MoS 3. Now we need to stay on-target with our Per-based roll, working against a penalty of -17 (-10 base, -7 speed). Let's say we end up with MoF 5. We add this to our previous result, for a total MoF 2. Now we need to average all our hits together to determine the actual hit pecentage. MoS 3 is 100%, as are MoS 2 and 1. MoS 0 is 95%, and MoF 1 and 2 are each 0%. 3.95/5=.79, for 79% hits. Were we firing at RoF 30, we'd be looking at 23 or 24 hits (average is 23.7). The big problems with this system are the need for multiple rolls, the need to calculate average number of hits, and, most importantly, a lack of Spread values for weapons - seriously, shotguns I got from Tactical Shooting, mounted machine guns from a mention in the other thread, and I have no clue about the spreads of other weapons (other than that they probably have Spread between these two). Next up, an option to randomize number of hits!