Help Me Understand; Ballistics question

[berettaprofessor]

I was looking today at 22LR on MidwayUSA and found some information I don't get. Aquila SuperExtra 22 LR 40 Grain Solid Point is listed at muzzle velocity of 1080 fps and Muzzle Energy 64 ft lbs. Federal Premium Gold Medal 22LR 40 Grain Lead Match is also listed at muzzle velocity of 1080 fps, but has a muzzle energy of 104 ft lbs.

How is that possible? Bullet's of the same mass, pushed to the same velocity, should have the same muzzle energy, shouldn't they? That's basic physics, right, F=MA? Is the website simply wrong? Were these measurements somehow specific to whatever different barrels these were taken in? School me please.

 

[Claude Clay]

would not be the 1st time a mistake made it to print.
otherwise it is a very special, new way to measure.

 

[bockja]

Looks like a misprint on Midways behalf regarding the Aguila ammo. Other sites have it listed at 100 ft-lbs.

 

[Texan Scott]

Not F=MA, (as you're not calculating ft-lbs of FORCE, but ft-lbs per SECOND of kinetic energy, so) E=M*V^2.

((1080fps^2) * (40gr)) divided by 450395, fixed unit conversion constant = 103.59 ft-lbs/sec KE at the muzzle.

Obvious manufacturer's error.

 

[Birch Knoll]

That's basic physics, right, F=MA? Is the website simply wrong?

As other have pointed out, 104 ft-lbs is correct. The other value is an error.

Muzzle energy isn't calculated as F=MA, however; it's the kinetic energy formula, Ke=1/2 mv^2.

 

[Skribs]

Whether you're looking at F=MA, or Ke=1/2mv^2, both show that the only variables are the mass and the velocity or acceleration, and that if mass and velocity are the same, the formula should equal out. So the OP is correct, it is a math equation, and those are the variables, and so the answer should be the same.

 

[Texan Scott]

wow... i totally said e=m*v^2, not 1/2m*v^2, didn't I? ... forgetting that my fixed constant is halved so i don't have to halve the mass...

I HAVE BECOME MY PHYSICS PROF.... I just worked part of the problem in my head, and forgot to mention it when working it for the class... :banghead:

ETA: "You'll need to correct my error in your notes...", he says.

 

[Stress_Test]

If you have the Excel program, try using this equation:

=((1/2)*(grains*0.0001429)*(vel)^2)/32.174


using input from a spreadsheet cell for "grains" and "vel".


The 0.0001429 is a conversion from grains to pounds.

If you try this equation you might wanna compare it against some stated manufacturers' specs to make sure you get the same answers in case I typo'd something there.

Then you can have your spreadsheet where all you have to do is enter the bullet weight in grains, and velocity in ft/sec, and poof, there's the ft-lbf for the round in question.

It's lots of fun to take this over to the Gunblast website and get ft-lbf numbers for all the different stuff he tests. Have fun!