(Bullet Knockback) What's wrong with this picture?

[Ulzgoroth]

Originally Posted by Tomsdad

I was talking about normal people, not impenetrable ones

Right, should have caught that looking back. That's even harder if you care about accuracy. How much momentum transfers when the projectile goes through? Do they explode from hydrostatic shock from such a large, fast projectile? What does that even look like, and how large and how energetic are the pieces?

You may have an easier time getting away with just going with whatever, since you're less likely to have any players invested in the details of the outcome...

[Varyon]

Using the guidelines from GULLIVER, with the test cases of 5.56x45mm, 7.62x39mm, and .50 BMG rounds, I previously determined that a bullet that fails to penetrate does 1/3rd of its piercing damage as crushing. Well, for pi anyway - every +2 SSR (2 steps on the Size and Speed/Range table) to wounding class is +1 SSR to the crushing component, to a maximum of 100% for pi+6 (roughly 70mm/2.75 caliber). Note this trend was based only on those three test cases, because I didn't feel like doing any further lookups and calculations.

When it comes to determining knockback of a bullet that does penetrate, first cap damage using the blowthrough rules from High Tech (but use penetrating damage, not injury). Add to this any penetrated DR. Calculate the Crushing component as above, and use that to determine if there's any appreciable knockback. Realistically, you should use the character's mass-based HP (including the mass of all carried gear), and should calculate knockback based on average rather than rolled damage (the latter more represents striking more or less vulnerable sublocations than actual variability in impact force) but for most games you'll probably be ok ignoring that.

So, for example, the M82A1 does 12d pi+. That averages 42 damage - dividing this by 2.5 works out to around 17. An unarmored man with HP 10 would take 10 damage before blowthrough, for 4 crushing - not enough to cause appreciable knockback. An HP 10 man with impenetrable skin (say, DR 100 vs pi/cut only) would take 42 damage, which would become 17 crushing (optionally causing injury, as his DR doesn't protect against crushing damage), and would knock him back 2 yards. This also implies that an ST 10 shooter (why are you using a .50 cal with ST 10?) would similarly be knocked back 2 yards. Keep in mind, however, that knockback is typically more stumbling than being tossed through the air, and that a properly-braced stance should be able to mitigate such issues. An ST 10 person firing an M82A1 from the hip without proper bracing strikes me as someone who is very likely to stumble back and fall on their butt.

[Bilanthri]

Originally Posted by Ulzgoroth

Except 'crushing cells' with shockwaves isn't terribly effective (people don't suffer massive necrosis from either bullet wounds or explosive blast-waves) and most tissue is elastic enough to snap back from the deformation...

"An 8-month study in Iraq performed in 2010 and published in 2011 reports on autopsies of 30 gunshot victims struck with high-velocity (greater than 2500 fps) rifle bullets.[25] In all 30 cases, autopsies revealed injuries distant from the main wound channel due to hydrostatic shock. The authors determined that the lungs and chest are the most susceptible to distant wounding, followed by the abdomen."

https://en.wikipedia.org/wiki/Hydros...hock_(firearms)

[Varyon]

Originally Posted by Bilanthri

"An 8-month study in Iraq performed in 2010 and published in 2011 reports on autopsies of 30 gunshot victims struck with high-velocity (greater than 2500 fps) rifle bullets.[25] In all 30 cases, autopsies revealed injuries distant from the main wound channel due to hydrostatic shock. The authors determined that the lungs and chest are the most susceptible to distant wounding, followed by the abdomen."

https://en.wikipedia.org/wiki/Hydros...ock_(firearms)

The full text of the referenced research article can be found here. Unfortunately, they fail to define (as far as I could see) what "distant lesions" are, and what criteria they are using to determine such are from hydrostatic shock and not something else (no control group is present), making it difficult to analyze their findings.
Also, I took the liberty of fixing your link to Wikipedia in my quote.
EDIT: All that said, this thread is about knockback, and hydrostatic shock is a different subject. You may wish to start a new thread for this tangent.

[Skarg]

I don't think that wounding damage does cover what I'm interested in, though. Taking HT/2 in damage and needing to roll HT or fall seems to me to represent falling down due to injury and shock.

What I'm interested in (though then it begins to be a different question which I started a thread about some time ago, which is an attack leading somehow to someone moving and/or falling, as demonstrated throughout that thread in various videos of people falling down and ending up several hexes away from the attack or initial mishap, generally without taking particularly much damage.

In this thread about this one bullet video though, it looks like there was at least as much force applied to the block as a (perhaps not very strong) shove which perhaps might usually not reliably knock someone back a hex, but if I were doing a pesky crunchy house rule for it, maybe should have a chance to. Then again, looking at the timing of the movement, as I mentioned above, it seems a bit like the movement of the block might have happened not due to the the bullet passing through, but by the dramatic cavitation/collapse/explosion and venting that happened, the physics of which I don't entirely understand.

[Tomsdad]

Originally Posted by Skarg

I don't think that wounding damage does cover what I'm interested in, though. Taking HT/2 in damage and needing to roll HT or fall seems to me to represent falling down due to injury and shock.

What I'm interested in (though then it begins to be a different question which I started a thread about some time ago, which is an attack leading somehow to someone moving and/or falling, as demonstrated throughout that thread in various videos of people falling down and ending up several hexes away from the attack or initial mishap, generally without taking particularly much damage....

I think the problem is that while GURPS splits thing into distinct things (injury, knockback) in reality all these things are on a spectrum and both things can apply. More over it also splits knock back and falling prone i.e. horizontal and vertical movement, as separate things* (even if one trigger the other). Similarly footing and surface.

I still pretty much stand by what I posted in that thread. But the point is reality doesn't work under separate distinct rules and hexes. And the rules are designed to work for a general group of things not hyper specific internet video instances.

However that said maybe some kind of glancing blow rule that models instances where attacks that are designed to inflict damage actually end up partially converting to physical movement. Something less a less conscious roll with blow. Of course different attacks would still work differently here (bullets vs cow kicks)

*although of course falling prone can mean falling into a adjacent hex

[Tomsdad]

Originally Posted by Varyon

The full text of the referenced research article can be found here. Unfortunately, they fail to define (as far as I could see) what "distant lesions" are, and what criteria they are using to determine such are from hydrostatic shock and not something else (no control group is present), making it difficult to analyze their findings.
Also, I took the liberty of fixing your link to Wikipedia in my quote.
EDIT: All that said, this thread is about knockback, and hydrostatic shock is a different subject. You may wish to start a new thread for this tangent.

Interesting (and yes i agree with your points)

[Ottriman]

A gameable abstraction I use that helps cover edge cases like nigh invulnerable men and such.

Burning: Burning from exposure to heat or being engulfed in hot things (boiling water, lava, being on fire etc etc) causes no knockback.

0,1x knockback for massless or near massless energy strikes like blaster or laser beams.

0,2x knockback for being struck my more substantial fire based attacks (Plasma guns, flamethrowers).

Cutting: Counts for knockback, but only the damage that fails to penetrate DR counts (not all or nothing).

Impaling: Half of the damage stopped by DR counts for knockback.

Piercing: Fraction of crushings knockback with only damage stopped by DR counting, multiplier depends on piercing size.

Pi-: 0,1
Pi: 0,2
Pi+: 0,3
Pi++: 0,4

Corrosive, Fatigue or Toxic have no knockback.

I also separate mass from hp, all creatures having mass points as part of their stats that replace hp for things like collisions, knockback resistance etc etc.

[Varyon]

Originally Posted by Skarg

I don't think that wounding damage does cover what I'm interested in, though. Taking HT/2 in damage and needing to roll HT or fall seems to me to represent falling down due to injury and shock.

More than HP/2, actually. And... that's sort of our point. "Knockdowns" from firearm wounds are due more to injury and shock than the actual force of the bullet.

Originally Posted by Skarg

What I'm interested in is an attack leading somehow to someone moving and/or falling, as demonstrated throughout that thread in various videos of people falling down and ending up several hexes away from the attack or initial mishap, generally without taking particularly much damage.

In GURPS, we call that a "push." It should be possible to mix-and-match "pushing" and "striking," such that someone who rolls 5 damage with a punch could have that be 2 normal damage and 3 pushing damage, for a total injury (assuming no armor) of 2 HP and total knockback of 8 (enough to push an HP 10 character back 1 yard), but that can be a bit finicky.

Originally Posted by Skarg

In this thread about this one bullet video though, it looks like there was at least as much force applied to the block as a (perhaps not very strong) shove which perhaps might usually not reliably knock someone back a hex, but if I were doing a pesky crunchy house rule for it, maybe should have a chance to.

That block looks (haven't done an actual measurement) to have a height around 10x the width of the bullet, and around 3x as thick as it is tall. Assuming it's the same width and height, that means it's around 4.4"x4.4"x13.2" - 4"x4"x12" seems more likely. That gives the block a weight right around 7 lb. Even ignoring that nifty cavitation explosion, do you think something that would flick a 7 lb object up slightly, just to have it land pretty much where it started out, would push a ~160 lb person back appreciably?

Originally Posted by Skarg

Then again, looking at the timing of the movement, as I mentioned above, it seems a bit like the movement of the block might have happened not due to the the bullet passing through, but by the dramatic cavitation/collapse/explosion and venting that happened, the physics of which I don't entirely understand.

From what I understand, here's how the cavitation explosion works. First, the bullet passing through the gel doesn't just leave a neat hole, it actually shreds some of the gel, leaving behind small particles of it. The large temporary cavity is a vacuum (because there was no air there before the cavity was opened), so air starts rushing in to fill it from the entry and exit holes. The temporary cavity is temporary, however, so it begins collapsing, and does so more quickly than the air can escape. Quickly compressing the air causes it to heat up markedly. This high heat, combined with the presence of oxygen, ignites the aerosolized gel particles (gelatin is flammable), essentially causing a small dust explosion inside of the gel. And, yeah, that explosion is probably more responsible for the gel tilting up than the bullet itself was.

I don't think this can happen inside of a person. First, our innards aren't quite the vacuum you see with a ballistics gel. Secondly, everything inside of us is a bit too wet to burn very readily. Third, and finally, I don't think the temporary cavities inside of a person are quite as dramatic as you see in a gel.

[Bruno]

Originally Posted by Hellboy

Seems like custom no-knockback rules wouldn't be needed if damage.was halved and just made it impaling. I realize bullets have round fronts but they seem to function in an impaley manner.

The distinction is very important for armor: a sufficient thickness of kevlar is great against bullets, and yet pretty crap against a crossbow bolt or even a thug wielding a "simple" knife.