McAnally's (The Community Pub) > Author Craft

Physics of A Firearm of Unusual Size

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Lanodantheon:
Writing a short story involving a 185ft Giant using an appropriately sized Dragonuv Sniper Rifle for vermin shootin'.


You can guess what kind....


But, I also wanted to know more about this normally cartoony situation. SO I found I had to use... *Dramatic swell*....MAAAATH.....*Orchestra Hit*....


Unfortunately, I'm not so good with physics and I just wanted to know more about the logistics of this situation from somebody who knows more than me. Hopefully somebody lurking this forum will know something and have a TI-83 handy.


Note: For reference I used a 2 inch tall ninja(5'5" in life) I have on my desk as reference compared to my 5'8" giant. And this assumes an appropriately scaled gun and full-metal jacket bullets. Assume he got the Dragonuv online at the giant supply store(he did actually now that I think of it).


1. How tall is the giant using my aforementioned scale? My initial calculation came to about 185ft tall give or take. 


2. How big would the bullets be, i.e. could a human sized person carry one? I already calculated a 7.62x56mm round scaled up to about a foot in diameter and 6 ft long but I don't know how much that would weigh.


3. Physics doesn't scale well against gravity and having 34 times the gunpowder does not mean that 34 times as much force will be imparted. How good would the muzzle velocity actually be assuming gunpowder? If he misses the bullets will rip through things, I'd just like to know how far it'll go until it stops and what will happen tot he ground it hits.


4. Assuming the scaled up bullets actually work(I don't want to handwave it but I can with Magic), how loud would the shots be? If it's loud enough, the heroes holding onto the giant may not need to worry about getting stepped on after being violently deafened by the sonic boom.

Aminar:
I would assume that your going to need to BS/fudge some of the physics anyway, as the materials a normal gun is made of wouldn't hold up well to that kind of abuse.  Maybe having it use a special form of gunpowder that is giant mixed and some explosion proofing of mythic proportions or the like. 

Basically literary fudging versus working the numbers out perfectly.

Haru:

--- Quote ---1. How tall is the giant using my aforementioned scale? My initial calculation came to about 185ft tall give or take. 
--- End quote ---
That's about what I got, too.


--- Quote ---2. How big would the bullets be, i.e. could a human sized person carry one? I already calculated a 7.62x56mm round scaled up to about a foot in diameter and 6 ft long but I don't know how much that would weigh.

--- End quote ---
The calculations for the size seem about right.
That would amount to about a ton (1000 kilograms) for 1 round. Kind of seems like overkill ;)
The calculation:
m=V*d
m = mass
V = volume
d = density (which for steel is 7.85 g/cm³)

I approximated the form of the bullet as a simple cylinder, which will be enough to ballpark a weight. The volume is then:
V=r²*PI*l
r = radius (1/2 foot in your case)
l = length (6 feet)


--- Quote ---3. Physics doesn't scale well against gravity and having 34 times the gunpowder does not mean that 34 times as much force will be imparted. How good would the muzzle velocity actually be assuming gunpowder? If he misses the bullets will rip through things, I'd just like to know how far it'll go until it stops and what will happen tot he ground it hits.
--- End quote ---
Actually, gravity is part of the whole physics thing. If you can get your ton of bullet up to the speed of a regular bullet, it would be affected by gravity like any normal bullet would. The only thing it would have to deal with would be greater aerodynamic resistance from the increased surface.

The formula here is:
F=m*a
F = Force
m = mass
a = acceleration

or the other way around:

a=F/m

which means, if you have double the mass, you need double the force to get it to an equal speed. I estimated the weight of the small bullet from your numbers above to about 20grams, which would mean you would need 50,000 times the force of a regular load to get the same result.


--- Quote ---4. Assuming the scaled up bullets actually work(I don't want to handwave it but I can with Magic), how loud would the shots be? If it's loud enough, the heroes holding onto the giant may not need to worry about getting stepped on after being violently deafened by the sonic boom.

--- End quote ---
I am not sure how much propellant you need to shoot a normal bullet, if you find out, multiply it by 50,000 and you will have an enormous pile of explosive material lying around. It might or might not be enough, but it is a good estimate, I think. I don't think it would create a sonic boom, because that would only happen if the bullet passed the sound barrier. The explosion would be loud, but manageable, I think.

Hope I could help so far.

Breandan:
It would, for simplicity's sake, be a rifle firing the shells used in US naval battleships, more or less. So, research those and you'll have an idea of what you are dealing with. Now, that being said... why in the nineteen flaming hells of tap-dancing China is he using THAT against people? Squish em, throw stuff at them, or simply roll gigantic rocks around on them. Much more effective :)

Aminar:

--- Quote from: Breandan on October 19, 2011, 04:57:47 AM ---It would, for simplicity's sake, be a rifle firing the shells used in US naval battleships, more or less. So, research those and you'll have an idea of what you are dealing with. Now, that being said... why in the nineteen flaming hells of tap-dancing China is he using THAT against people? Squish em, throw stuff at them, or simply roll gigantic rocks around on them. Much more effective :)

--- End quote ---

On that point, at this size solid chunks of metal cause explosions(frag ones at least) on impact, are you taking that into account? Because using it as a varmint rifle seems silly when he leaves craters as big as his feet after every shot and obliterates elephants.
That said-Written well I can see this being pretty damned cool.

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