Another angle on equivalent energy of pneumatic tests

[robsalv]

I've seached all the good stuff in Eng-tips about the equivalent energy in a pnuematic test. Fascinating stuff.

A friend of mine was a bystander when someone's pnuematic test went wrong. Luckily it was only a small volume. Based on the information in these forums, I calculated the test was equivalent to 2grams of TNT.

Doesn't sound like much, but I have no experience with explosives.

To help me put this figure in perspective, I was trying to find out the typical energy/grams of propelant/equivalent grams of TNT etc used in a typical .22 rifle bullet - unfortunately, proxy surfing at work has sites like that barred.

Does anyone know how much energy is in a typical rifle bullet?

Thanks in advance.

Rob

 

[Scipio]

Never really thought of it before, but the question got me poking around the website - if you can get it away from work, do a websearch for 'internal ballistics', actually some pretty interesting stuff out there. Looks like the actual energy varies a lot, and for hand/shoulder arms seems to be largely based on test data rather than calculations - something to do with impredictabilities on such a small scale, larger scale ballistic weapons, like say tank guns, can be calculated.

That said, it looks like the average peak pressure after firing varies anywhere from about 40,000 psi to 60,000 psi, depending on a lot of things including type of powder, how much was used, and the ratio of mass of powder to mass of the bullet. Also, the amount of energy produced during detonation which goes into actually moving the bullet varies a lot, anywhere from 15% to 35% efficiency.

Doing a couple quick hand calcs with some ballistic tests I found on the internet, I took a .25 calibre, 75 grain bullet test fire using a 25 grain charge. Estimated muzzle velocity was about 2800 feet per second. If I've got the conversion right, there's 7000 grains per pound, so the mass of the bullet is about 0.0107 lbs (4.86 grams). Working out the kinetic energy of that bullet at muzzle velocity, E=1/2*m*V², that works out to about 41000 ft-lbs. Further, if we estimate the ignition was about 25% efficient transferring energy to the bullet, then the energy of the original detonation is about 160000 ft-lbs, from 1.62 grams of "Class 11, IMR 3031" powder, whatever that means!

 

[robsalv]

Thanks Scipio. I have no idea whether your numbers "look" right, but they're certainly impressive.

So the propellant load of a typical rifle bullet is about a 3rd of the bullet, i.e. 1.62g in your example. Your calcs would seem to suggest that ammo propellant is significantly more energetic than TNT which is an interesting turn up. [TNT has 4680J/gram] Not sure what I was expecting - but since the curiousity switch has been turned on, I'm interested to learn more.


Rob

 

[arto]

For a really interesting/historical look at this subject, check out J.S. Hatcher, "Hatcher's Notebook," Stackpole Books; 3rd edition (December 1962).
Lotsa good info - no BS. He was in charge of the Springfield Armory & had >50years of small arms/ammo design engineering experience.

 

[Scipio]

Interesting point, arto, a couple of the websites I'd been looking at kept referring to Hatcher's, but never really mentioned what it was.