I originally posted this in the Head Hit: Pilot Injured thread. Reposting by request and with additional clarifications.
TL;DR version
LRMs use a special clustering mechanic that causes missiles to clump together, but also prevents them from hitting the head unless the cluster lands on the head. If the cluster does hit the head, there is a chance that additional missiles will also hit the head, but the odds are very low.
SRMs do not use the clustering mechanics. Instead they are treated like firing a separate weapon for each missile, each with a separate chance to hit the head. That means that an SRM 6 gets 6 chances per volley to hit the head.
Machine guns work the same way as SRMs. Each of their 5 shots is independent and each has a chance to hit the head.
Regardless of how many weapons hit the head in a single mech's activation, it will only cause one pilot injury. Ammo explosions and losing side torsos are also included in this, but falls are resolved separately. As examples, if a mech loses a side torso from an ammo explosion and the pilot is hit by another weapon at the same time, they will only receive a single injury. If that same round of firing was also enough to knock the mech over, either from stability damage or loss of a leg, the pilot will take a second injury from the fall.
Long Version
I ran some LRM clustering tests to check how the clustering mechanic and head hits work.
For LRMs the first missile determines the center of the cluster. If it hits the head, then additional missiles can hit the head, but there is only 1 pilot wound. If the first missile hits anywhere other than the head, there is zero chance for other missiles in that cluster to hit the head. Each LRM rack has about a 1% chance of generating a head hit per volley. I tested this with 1,600 missiles fired across 16 volleys, half with standard hit tables and half with modified hit tables to guarantee head hits.
I have not tested indirect LRM fire to see if it uses a different hit table. I also haven't tested the accuracy of the hit tables to see if the RNG is producing more head hits than expected. There is some evidence that the actual hit rate is closer to 2% rather than the 1.2% predicted by the hit tables. Thank you to @Diedel for additional testing.
Clustering Explanation
All hit locations are determined by the hit locations tables in the CombatGameConstants JSON file. These provide the weight and probabilities of hitting each location.
This is the standard front hit location table. This shows the odds of hitting a location on a mech if you attack it from the front arc.
LRMs only use the standard tables to determine the first missile hit location which determines the center of the cluster. The location hit has it's weight multiplied by 8 and the adjacent locations are multiplied by 4, with one exception. The head is not multiplied if the cluster lands on it and is set to 0 if the cluster isn't centered on the head.
This is what the hit location table looks like if the cluster is centered on the head:
Notice that the head hit odds actually went down for subsequent missiles. That is because the head isn't multiplied, but it is still adjacent to the torsos, so those do get their odds multiplied. The odds for the initial hit are still 1 out of 81, but don't expect to be getting multiple LRMs to hit the head from a single launcher.
As further examples here are the Center Torso and Left Leg cluster tables:
Also of note, called shots only affect where the cluster is centered. They don't modify the cluster hit tables. An LRM 20 with a called shot to the CT still only expects 9 or 10 missiles to hit the CT.
Testing Methodology
I tested clustering and LRM head hits in the single player skirmish against target dummies, Atlai with 1,000 armor in every location but only 2 medium lasers.
I modified an LRM 5 to fire 100 missiles per volley to get clear clusters and set the damage to 1 per missile with no stability damage. I didn't want to knock them over. I was just using armor damage to show hit locations.
I took 4 Kintaros firing only their single LRM 5 each round. I did two rounds with unmodified hit charts and had no head hits.
Results with unmodified hit charts:
Where each cluster landed should be clear and even over 800 missiles there were no head hits.
After that I modified the forward hit charts to guarantee that I would get some head hits.
That gives a 40% chance for any attack to hit the head. I actually had to do 9 shots on this test because the 8th shot got the 4th head head and killed the pilot. With the mech down I couldn't check the armor. This also shows that only 1 pilot injury is generated per volley, regardless of how many missiles hit the head.
Results:
As you can see, the volleys that weren't centered on the head had zero head hits, even with the increased head hit chance.
I also did a quick SRM test with the modded hit tables before switching it back. I only fired 2 volleys, but with 44% head hits on the first shot and 41% on the second, it was pretty clear that they are not using the clustering mechanic that LRMs are. SRMs roll location per missile with no modifications for clustering, meaning each missile, not each missile rack, has about a 1% head hit chance.
Machine guns work the same. I just did a test with 100 shot, 1 damage machine guns on unmodified hit tables. There was no clear clustering and each of the 3 shots I fired had head hits. Out of 300 total rounds, there were 7 head hits for a hit rate of 2.3%. That is actually higher than I would expect as the rate should be 1.23% (1 out of 81).
EDIT: Keeping head hit and pilot injury separate can be tricky.
EDIT2: Adding details on how LRMs function with called shots. TL;DR They still use the clustering mechanics. The called shot should be weighting where the cluster lands, but has no further impact.
TL;DR version
LRMs use a special clustering mechanic that causes missiles to clump together, but also prevents them from hitting the head unless the cluster lands on the head. If the cluster does hit the head, there is a chance that additional missiles will also hit the head, but the odds are very low.
SRMs do not use the clustering mechanics. Instead they are treated like firing a separate weapon for each missile, each with a separate chance to hit the head. That means that an SRM 6 gets 6 chances per volley to hit the head.
Machine guns work the same way as SRMs. Each of their 5 shots is independent and each has a chance to hit the head.
Regardless of how many weapons hit the head in a single mech's activation, it will only cause one pilot injury. Ammo explosions and losing side torsos are also included in this, but falls are resolved separately. As examples, if a mech loses a side torso from an ammo explosion and the pilot is hit by another weapon at the same time, they will only receive a single injury. If that same round of firing was also enough to knock the mech over, either from stability damage or loss of a leg, the pilot will take a second injury from the fall.
Long Version
I ran some LRM clustering tests to check how the clustering mechanic and head hits work.
For LRMs the first missile determines the center of the cluster. If it hits the head, then additional missiles can hit the head, but there is only 1 pilot wound. If the first missile hits anywhere other than the head, there is zero chance for other missiles in that cluster to hit the head. Each LRM rack has about a 1% chance of generating a head hit per volley. I tested this with 1,600 missiles fired across 16 volleys, half with standard hit tables and half with modified hit tables to guarantee head hits.
I have not tested indirect LRM fire to see if it uses a different hit table. I also haven't tested the accuracy of the hit tables to see if the RNG is producing more head hits than expected. There is some evidence that the actual hit rate is closer to 2% rather than the 1.2% predicted by the hit tables. Thank you to @Diedel for additional testing.
Clustering Explanation
All hit locations are determined by the hit locations tables in the CombatGameConstants JSON file. These provide the weight and probabilities of hitting each location.
This is the standard front hit location table. This shows the odds of hitting a location on a mech if you attack it from the front arc.
LRMs only use the standard tables to determine the first missile hit location which determines the center of the cluster. The location hit has it's weight multiplied by 8 and the adjacent locations are multiplied by 4, with one exception. The head is not multiplied if the cluster lands on it and is set to 0 if the cluster isn't centered on the head.
This is what the hit location table looks like if the cluster is centered on the head:
Notice that the head hit odds actually went down for subsequent missiles. That is because the head isn't multiplied, but it is still adjacent to the torsos, so those do get their odds multiplied. The odds for the initial hit are still 1 out of 81, but don't expect to be getting multiple LRMs to hit the head from a single launcher.
As further examples here are the Center Torso and Left Leg cluster tables:
Also of note, called shots only affect where the cluster is centered. They don't modify the cluster hit tables. An LRM 20 with a called shot to the CT still only expects 9 or 10 missiles to hit the CT.
Testing Methodology
I tested clustering and LRM head hits in the single player skirmish against target dummies, Atlai with 1,000 armor in every location but only 2 medium lasers.
I modified an LRM 5 to fire 100 missiles per volley to get clear clusters and set the damage to 1 per missile with no stability damage. I didn't want to knock them over. I was just using armor damage to show hit locations.
I took 4 Kintaros firing only their single LRM 5 each round. I did two rounds with unmodified hit charts and had no head hits.
Results with unmodified hit charts:
Where each cluster landed should be clear and even over 800 missiles there were no head hits.
After that I modified the forward hit charts to guarantee that I would get some head hits.
That gives a 40% chance for any attack to hit the head. I actually had to do 9 shots on this test because the 8th shot got the 4th head head and killed the pilot. With the mech down I couldn't check the armor. This also shows that only 1 pilot injury is generated per volley, regardless of how many missiles hit the head.
Results:
As you can see, the volleys that weren't centered on the head had zero head hits, even with the increased head hit chance.
I also did a quick SRM test with the modded hit tables before switching it back. I only fired 2 volleys, but with 44% head hits on the first shot and 41% on the second, it was pretty clear that they are not using the clustering mechanic that LRMs are. SRMs roll location per missile with no modifications for clustering, meaning each missile, not each missile rack, has about a 1% head hit chance.
Machine guns work the same. I just did a test with 100 shot, 1 damage machine guns on unmodified hit tables. There was no clear clustering and each of the 3 shots I fired had head hits. Out of 300 total rounds, there were 7 head hits for a hit rate of 2.3%. That is actually higher than I would expect as the rate should be 1.23% (1 out of 81).
EDIT: Keeping head hit and pilot injury separate can be tricky.
EDIT2: Adding details on how LRMs function with called shots. TL;DR They still use the clustering mechanics. The called shot should be weighting where the cluster lands, but has no further impact.
Testing Methodology
I used Kintaros shooting at Atlas target dummies in skirmish. The Atlai had no weapons and max armor. The Kintaros were loaded with knockdown SRMs (0 damage, 50 stability damage) and an LRM 100 with no change to damage. I made the absolute minimum changes I need to test this. I would knock down an Atlas with 2 of the Kintaros, record its starting armor values, fire with just 1 LRM 100, recording hit odds and called shot odds as reported by the game, then counted how many hits occurred at each location.
Background Info
Here are the cluster hit tables for the center torso, right side, and right leg. These are just the standard prone hit table, as located in the CombatConstants JSON, multiplied by the cluster hits modifiers (x8 for center of cluster, x4 for adjacent, and x1 for all others, with 0 for head).
Results
Here are the results from 10 shots. I mostly called the CT and 3 of my 4 mechwarriors had Tactics of 6+, so they had the first called shot skill bonus. I did call shots on a couple other locations to get some more info and to avoid coring the Atlai too quickly. Each Atlas carries 320 CT armor + 160 CT structure. After 2 volleys at full damage they were going internal and the third CT cluster finished each one off.
NOTE: Each shot of 100 LRMs was dealing about 200 damage to the CT. Two shots was putting an Atlas at about 100 structure left on the CT. If that seems like a lot, remember that I was firing 100 LRMs in each volley and with full damage each (no cover/brace). The standard complement of an assault LRM boat is 60 LRMs (4x LRM 15 Stalker, 3x LRM 20 Highlander). That is taking the full firepower of 3 assault mechs just to go internal on a max armor Atlas.
There was one instance where I called a shot on the arm and the cluster missed, hitting center torso instead. That Atlas had already taken 2 clusters to the CT and was destroyed. Unfortunately, I did not record all of the damage, but I do know that it would have take at least 60 missiles to destroy the CT and I don't think there were many, if any hits to the arm that I called the shot on. I did get the info for the last mech I destroyed and am only missing the exact number of missiles that hit the CT (but I do have the minimum).
They are all fairly reasonably close to their appropriate cluster hits table. The most interesting result was when I did a called shot on the head. Naturally, the cluster didn't land on the head (it was only a 2% chance) so there were no hits to the head. If called shots from LRMs bypassed the clustering mechanics and used the called shot tables, I would expect the hits to be fairly evenly distributed across the mech.
Instead it landed on the right torso. The hits are very clearly weighted in favor of the right side. They are perhaps a little heavier on the CT than I would expect, but some variance is normal. (heh, normal... unintended statistics pun...)
I agree that the cluster hits tables do resemble the called shot tables. They both weight a specific location more than the others. The use of the clustering tables is probably clearest in looking at the instances when the cluster does not land on the called location. It would also be clearer with Called Shot Mastery as that would give 90% odds for the cluster to hit the CT... but only 55-65% of the missiles would hit the CT.
Conclusion
LRMs use the clustering mechanics, even when fired with a called shot. They deal a lot of damage per ton, but it is unfocused and generally bad at destroying fully armored locations.
I used Kintaros shooting at Atlas target dummies in skirmish. The Atlai had no weapons and max armor. The Kintaros were loaded with knockdown SRMs (0 damage, 50 stability damage) and an LRM 100 with no change to damage. I made the absolute minimum changes I need to test this. I would knock down an Atlas with 2 of the Kintaros, record its starting armor values, fire with just 1 LRM 100, recording hit odds and called shot odds as reported by the game, then counted how many hits occurred at each location.
Background Info
Here are the cluster hit tables for the center torso, right side, and right leg. These are just the standard prone hit table, as located in the CombatConstants JSON, multiplied by the cluster hits modifiers (x8 for center of cluster, x4 for adjacent, and x1 for all others, with 0 for head).
Results
Here are the results from 10 shots. I mostly called the CT and 3 of my 4 mechwarriors had Tactics of 6+, so they had the first called shot skill bonus. I did call shots on a couple other locations to get some more info and to avoid coring the Atlai too quickly. Each Atlas carries 320 CT armor + 160 CT structure. After 2 volleys at full damage they were going internal and the third CT cluster finished each one off.
NOTE: Each shot of 100 LRMs was dealing about 200 damage to the CT. Two shots was putting an Atlas at about 100 structure left on the CT. If that seems like a lot, remember that I was firing 100 LRMs in each volley and with full damage each (no cover/brace). The standard complement of an assault LRM boat is 60 LRMs (4x LRM 15 Stalker, 3x LRM 20 Highlander). That is taking the full firepower of 3 assault mechs just to go internal on a max armor Atlas.
There was one instance where I called a shot on the arm and the cluster missed, hitting center torso instead. That Atlas had already taken 2 clusters to the CT and was destroyed. Unfortunately, I did not record all of the damage, but I do know that it would have take at least 60 missiles to destroy the CT and I don't think there were many, if any hits to the arm that I called the shot on. I did get the info for the last mech I destroyed and am only missing the exact number of missiles that hit the CT (but I do have the minimum).
They are all fairly reasonably close to their appropriate cluster hits table. The most interesting result was when I did a called shot on the head. Naturally, the cluster didn't land on the head (it was only a 2% chance) so there were no hits to the head. If called shots from LRMs bypassed the clustering mechanics and used the called shot tables, I would expect the hits to be fairly evenly distributed across the mech.
Instead it landed on the right torso. The hits are very clearly weighted in favor of the right side. They are perhaps a little heavier on the CT than I would expect, but some variance is normal. (heh, normal... unintended statistics pun...)
I agree that the cluster hits tables do resemble the called shot tables. They both weight a specific location more than the others. The use of the clustering tables is probably clearest in looking at the instances when the cluster does not land on the called location. It would also be clearer with Called Shot Mastery as that would give 90% odds for the cluster to hit the CT... but only 55-65% of the missiles would hit the CT.
Conclusion
LRMs use the clustering mechanics, even when fired with a called shot. They deal a lot of damage per ton, but it is unfocused and generally bad at destroying fully armored locations.
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