Science-based what-if questions

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Sableagle
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Re: Science-based what-if questions

Postby Sableagle » Fri Jun 08, 2018 3:58 pm UTC

Ah, well, if you form an airtight seal you've got the air already in the barrel cushioning your hand, haven't you?

580 MPa? Well, that's less than the 163 GPa I worked out for that 9mm bullet stop. (Only 2 GPa if you allow it 4mm of skin thickness in which to stop).

The amount of propellant in the tank round outweighs the air in the barrel, and the propellant gas gets hot, so the shell will go most of the way before the air being compressed in front of it exerts as much pressure on its front as the propellant gas does on its rear, but that'll happen somewhere. I doubt the compression heating would set the shell off, but the pressure between the shell and your fist could get high enough to pop the barrel wide open and wreck the gun ... if you're super-tough and don't get popped out like a champagne cork, of course.

After all, guns are only made to hold so much pressure, the pressure decreases as the bullet goes down the barrel (beyond where the powder finishes burning fast enough to keep it up) in normal use and they don't like being obstructed. Of course, water translates impact into outward pressure pretty well.

6.56 MN. PI * (60mm)2 * 580 MPa = 6.56 MN. That's like the weight of 670 metric tonnes.

Here's a 670 ton displacement barque called Alexandrina. Could this Blob guy lift that one-handed?
Oh, Willie McBride, it was all done in vain.

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Re: Science-based what-if questions

Postby Tyndmyr » Tue Jun 12, 2018 9:15 pm UTC

Sableagle wrote:Ah, well, if you form an airtight seal you've got the air already in the barrel cushioning your hand, haven't you?

580 MPa? Well, that's less than the 163 GPa I worked out for that 9mm bullet stop. (Only 2 GPa if you allow it 4mm of skin thickness in which to stop).

The amount of propellant in the tank round outweighs the air in the barrel, and the propellant gas gets hot, so the shell will go most of the way before the air being compressed in front of it exerts as much pressure on its front as the propellant gas does on its rear, but that'll happen somewhere. I doubt the compression heating would set the shell off, but the pressure between the shell and your fist could get high enough to pop the barrel wide open and wreck the gun ... if you're super-tough and don't get popped out like a champagne cork, of course.

After all, guns are only made to hold so much pressure, the pressure decreases as the bullet goes down the barrel (beyond where the powder finishes burning fast enough to keep it up) in normal use and they don't like being obstructed. Of course, water translates impact into outward pressure pretty well.

6.56 MN. PI * (60mm)2 * 580 MPa = 6.56 MN. That's like the weight of 670 metric tonnes.

Here's a 670 ton displacement barque called Alexandrina. Could this Blob guy lift that one-handed?



That is not a lot of air, and anyways, if it were compressing enough to stop a bullet, it would be exerting equal force upon your finger. Your finger being more fragile than a lead projectile, it is likely that your finger is going to give first. Not sure of the exact math for comic book physics, but a real world finger in a real world gun, your finger will lose. Depending on precise load of round and amount of tolerance in the gun barrel, the added strain *may* damage the gun, but that will not matter much to your hand. This is vastly unlikely, as most guns are made with very high tolerances because, yknow, gun.

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Re: Science-based what-if questions

Postby SuicideJunkie » Wed Jun 13, 2018 2:55 pm UTC

If your finger/arm deforms to seal well, doesn't leave the barrel or leak air, then the pressure and force will rise towards infinity as the fingertip/hand gets deformed to match the shape of the bullet/shell and the remaining volume of barrel-air drops.

An explosive shockwave and crushed finger/limb that would ruin you, are merely laughable flesh wounds in comic physics. (Note that the plugger isn't looking at the explosion, which makes them cool and fine.) Shake off the coal dust and tingles by face-punching more bad guys.

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Re: Science-based what-if questions

Postby p1t1o » Wed Jun 13, 2018 3:33 pm UTC

In high-energy environments, flesh can be approximated with water. If you block the barrel of a gun with just a few inches of water, there is a good chance of barrel rupture - plenty of empirical evidence for this on youtube.
Ergo, an arm/finger in the barrel will certainly get smashed to poop, but the gun may not come out unscathed either.
Depends on the weapon of course.

Tyndmyr wrote: it is likely that your finger is going to give first.


Not necessarily, your finger has mass, so to get it out of the way fast enough it must be accelerated very hard - significantly harder than the bullet has been accelerated. So whilst the forces may be unbalanced and acting to push the finger out of the way, the finger may not accelerate fast enough.

Like if I took a door off its hinges and stood it up on the floor, balanced on one of its edges, you could push it over with only a few Newtons of force. But if you sprinted at it as fast as you could and ran straight into it, you would get an awful whack of many Newtons - then the door would fall down.

So you might get: insert finger - bullet fired - air in barrel compresses - finger starts to move backwards - bullet continues travelling, pressure continues to rise - barrel ruptures - finger (or finger-mush) exits barrel (or whats left of it).

***

This is a real danger in firearms maintenance - a small piece of fabric wadding left stuck in a barrel can be pushed out with a rod, but stands a very good chance of explosively dismantling the weapon if it is fired in this state.

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Re: Science-based what-if questions

Postby Tyndmyr » Wed Jun 13, 2018 3:37 pm UTC

Oh yes, the finger chunks will exit the barrel, that's not the contention. The point is that they'll not stop the bullet. That's the essential bit right there for the future of said finger.

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Re: Science-based what-if questions

Postby p1t1o » Wed Jun 13, 2018 3:46 pm UTC

I was concentrating on the fate of the weapon but you are right of course. Bullet will already be moving and there only one way to stop it.

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Re: Science-based what-if questions

Postby Soupspoon » Wed Jun 13, 2018 3:55 pm UTC

p1t1o wrote:Like if I took a door off its hinges and stood it up on the floor, balanced on one of its edges, you could push it over with only a few Newtons of force. But if you sprinted at it as fast as you could and ran straight into it, you would get an awful whack of many Newtons - then the door would fall down.

What if I paint the door onto a brick wall? Will it work the same as painting a tunnel onto a rock wall?

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Re: Science-based what-if questions

Postby p1t1o » Wed Jun 13, 2018 5:09 pm UTC

Soupspoon wrote:What if I paint the door onto a brick wall? Will it work the same as painting a tunnel onto a rock wall?


These appear to be opposite cases, only one of them will resemble the previous example.

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Re: Science-based what-if questions

Postby Eebster the Great » Thu Jun 14, 2018 12:36 am UTC

It depends on if you paint hinges and on which direction you paint the latch.


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