Combining and Layering Flexible Armor Against Firearms

[General Lee]

Hi everyone!

I was reading this when I decided to check how gurps would deal it - yeah, I did not have it by rote. By the rules in Basic Set: Characters, 4ed, p. 286 armor could be combined in layers, if inner layers are flexible and concealable, adding DR from each other, with penalty of -1 per layer added.

So, in the example explained there, the armor combined (NIJ Ratings IIA+IIIA+~IIIA+~IIIA) would have ~DR 44, tested against a .300 AAC Sellier & Bellot 147 gr FMJ cartridge shot by an AR Rifle -- with muzzle energy roughly equal to "a 7.62x51mm M80 FMJ fired from an M14 at about 400 yards", by which for GURPS means nothing because 400 yards is below half-damage range --, which I estimated it doing 5d+2 of damage (or 20 Hit Points on average), and clearly it fails to protect a wannabe wearer of this armor. Why?

How come 20 Hit Points could blowthough an armor with ~DR44? My immediate answer to that -- IT COULDN'T, according to the rules as it is.

So I come up with this House Rule, against firearms only, layering soft armor would add only 20% of the original DR layer, round down, added to the highest layer DR. So, in the example above, the armor combined would have DR17, easy to a .300BLK, or any other rifle-powered cartridges, to overcome.

So, any ideas, is this the right direction, any comments, would you do it differently?

Thanks in advance.

[cdru]

Originally Posted by Hellboy

What if you go with maximum damage instead of average? 5D+2 would be 32... or 34 if you made an all-out attack.

Ranged weapons don't get damage bonuses for All-Out Attack

Anyways, there are few ways to penetrate: Shoot at armor chinks, OR get a critical hit

[acrosome]

My response is that calling the two IBA groin panels "level IIIA" is probably optimistic. I'd look into that a bit, first. But generally, yeah, soft armor is not meant to stop rifle bullets.

GURPS is a model, and to be playable it cannot be comprehensive to reality. If it were, what you'd have is some sort of AI reality simulation, and one combat second would take 32 years to play out. So there are going to be corners where it fails spectacularly. (Another is attacks against some structures and ships, which are WAY too easy to destroy in GURPS. There are others.) I'd wager that someone wearing four sets of Point Blank were not considered in the playtest.

[Varyon]

From what I understand, penetrated armor protects much less than GURPS suggests. A simplified version is that the square of armor protects against the square of damage - so if those armors were, say, DR 8, 12, 12, and 12, that's not DR 44, it's DR (64+144+144+144)^(1/2)=22, which still should have been enough to stop the 20 damage of the shot - it's possible our DR values are off, or the damage is off. A gameable version of this rule is to state that, if damage is greater than DR, divide DR by 3 to determine how much it reduces damage by. Above, we'll round that DR 8 up to DR 9, in which case the first layer is penetrated (17 damage remaining of our 20), as is the second (13 remains) and third (9 remains), but the fourth is enough to stop the bullet. Something is definitely off, particularly with the fact that the bullet apparently had enough chutzpah left to punch through an estimated 20 inches of flesh - which works out to right around 20 damage on its own (flesh is roughly DR 1/inch). That implies that, if the bullet can get through soft armor, it outright ignores it, rather than just reducing it via a difference of squares approach (or the simplified divide by 3 method, which tends to give very similar results). That, or the bullet in question has some sort of armor divisor to it.

[weby]

Soft ballistic armor works a lot different in real life than in Gurps. In Gurps armor reduces damage by a given value and thus the object on the other side takes less damage. In real life soft ballistic armor that fails to stop a projectile will tend to "catastrophically fail" at the location of the penetration and not slow down the projectile much thus it would hit a second such panel with most of it's energy left. Only if the penetration is really marginal can we see a large slowdown but penetration like the Gurps armor model.

To model the real behavior Gurps you would need to give such armors a fairly high "penetration value"(PW) and really low(or no) DR. The PW would then be compared to the damage and if damage was less then no penetration happens and blunt trauma is applied. If the damage was more then only the minor DR is applied and the resulting damage is applied to the object protected or inner layer armor. If doing such penetration value thing it would likely make mostly sense to use armor as dice type thing for the PWs and express them in dice of damage and not damage values.

[General Lee]

Originally Posted by acrosome

My response is that calling the two IBA groin panels "level IIIA" is probably optimistic. I'd look into that a bit, first. But generally, yeah, soft armor is not meant to stop rifle bullets.

GURPS is a model, and to be playable it cannot be comprehensive to reality. If it were, what you'd have is some sort of AI reality simulation, and one combat second would take 32 years to play out. So there are going to be corners where it fails spectacularly. (Another is attacks against some structures and ships, which are WAY too easy to destroy in GURPS. There are others.) I'd wager that someone wearing four sets of Point Blank were not considered in the playtest.

Definitely this case is in this pitfall, but the beauty of the system is that GMs are not required to follow ALL book rules by rote to all situations encountered. I was wandering a scenario like the North Hollywood Shootout for a Cops or SWAT Campaign with the adversaries with homemade armor, when I read the article.

Originally Posted by Varyon

From what I understand, penetrated armor protects much less than GURPS suggests. A simplified version is that the square of armor protects against the square of damage - so if those armors were, say, DR 8, 12, 12, and 12, that's not DR 44, it's DR (64+144+144+144)^(1/2)=22, which still should have been enough to stop the 20 damage of the shot - it's possible our DR values are off, or the damage is off. A gameable version of this rule is to state that, if damage is greater than DR, divide DR by 3 to determine how much it reduces damage by. Above, we'll round that DR 8 up to DR 9, in which case the first layer is penetrated (17 damage remaining of our 20), as is the second (13 remains) and third (9 remains), but the fourth is enough to stop the bullet. Something is definitely off, particularly with the fact that the bullet apparently had enough chutzpah left to punch through an estimated 20 inches of flesh - which works out to right around 20 damage on its own (flesh is roughly DR 1/inch). That implies that, if the bullet can get through soft armor, it outright ignores it, rather than just reducing it via a difference of squares approach (or the simplified divide by 3 method, which tends to give very similar results). That, or the bullet in question has some sort of armor divisor to it.

For sure I could say that reviewing Douglas Cole’s blog, I found stats for .300 AAC, and for an approximately 14.5 inch barrel, the damage is actually 5d, not 5d+2, as I put above. So the damage is definitely off. AFAIK the cartridge described would not have any modifiers, as armor divisor, in GURPS.

Your approach is appealing, still it not answers what happen. I was wondering if the armor used in the “test” were in good shape, i.e., could have its performance degraded by lower HT?

[Anthony]

The problem with the GURPS damage model is that it's trying to use a single DR value for multiple different armor and weapon types. This is an understandable simplification, anything realistic is going to be hell of complicated, but it means you get anomalies. Notably, flexible armor has somewhat different thickness scaling than hard armor, and also has variable protection with projectile velocity. A truly realistic assertion for the protection of level IIIa armor is that it stops the attacks it's rated as stopping, and no reliable statements can be made about its performance against other attacks.

[General Lee]

Originally Posted by weby

Soft ballistic armor works a lot different in real life than in Gurps. In Gurps armor reduces damage by a given value and thus the object on the other side takes less damage. In real life soft ballistic armor that fails to stop a projectile will tend to "catastrophically fail" at the location of the penetration and not slow down the projectile much thus it would hit a second such panel with most of it's energy left. Only if the penetration is really marginal can we see a large slowdown but penetration like the Gurps armor model.

To model the real behavior Gurps you would need to give such armors a fairly high "penetration value"(PW) and really low(or no) DR. The PW would then be compared to the damage and if damage was less then no penetration happens and blunt trauma is applied. If the damage was more then only the minor DR is applied and the resulting damage is applied to the object protected or inner layer armor. If doing such penetration value thing it would likely make mostly sense to use armor as dice type thing for the PWs and express them in dice of damage and not damage values.

Originally Posted by Anthony

The problem with the GURPS damage model is that it's trying to use a single DR value for multiple different armor and weapon types. This is an understandable simplification, anything realistic is going to be hell of complicated, but it means you get anomalies. Notably, flexible armor has somewhat different thickness scaling than hard armor, and also has variable protection with projectile velocity. A truly realistic assertion for the protection of level IIIa armor is that it stops the attacks it's rated as stopping, and no reliable statements can be made about its performance against other attacks.

So twisting both your point of views, in risk of oversimplifying an already oversimplified rule, I could say that against successful firearms (piercing) attacks, i.e., attacks that cause more damage than armor DR individually considered, soft armor has effective DR of a determined value – be it [sqrt(DR)] or 15-20% DR, like an extreme type of ablative armor limitation?
All that for sake of playability with a sense more accurate sense of reality, instead of ruling that “as this situation is beyond the curve of the system, I blinded myself and accept in play an unrealistic result”.

[General Lee]

Originally Posted by Hellboy

What if you go with maximum damage instead of average? 5D+2 would be 32... or 34 if you made an all-out attack.

As cdru answered, with ranged attacks (like firearms), AoA only have determined for +1 to hit a single attack, and Suppression Fire options.

Maximun damage could happen in critical hits or pure luck in dice, but it could not be considered as for rating armor.

Originally Posted by Hellboy

How are you estimating the damage dice?

In Adjusting Damage, GURPS High Tech 4ed., p.166, the first step is multiply damage dice by 3.5, retaining fractions, and add the damage bonus or penalty, before apply any modifier due to upgrades or options. Also, look this blog by HANS, it could help explain why I estimated damage this way.

Originally Posted by Hellboy

What page is this example on? I'm not seeing it on 286

The example is in the link that I post it, not in the p. 286. Sorry if I was not clearer before.

[Anthony]

Originally Posted by General Lee

So twisting both your point of views, in risk of oversimplifying an already oversimplified rule, I could say that against successful firearms (piercing) attacks, i.e., attacks that cause more damage than armor DR individually considered, soft armor has effective DR of a determined value – be it [sqrt(DR)] or 15-20% DR, like an extreme type of ablative armor limitation?
All that for sake of playability with a sense more accurate sense of reality, instead of ruling that “as this situation is beyond the curve of the system, I blinded myself and accept in play an unrealistic result”.

You can get playable results by just taking the split DR on flexible armors and saying the lower value applies to any weapon that isn't a pistol, shotgun, or explosive fragmentation.