[divegeek]

I wish I owned a couple of acres so that I could just go out and do the tests myself...

I own some land. What kind of tests would you do?

 

[divegeek]

One student of mine who carries a 9mm was recently told that his 9mm was simply a 45 set on "stun". (The commentator however, declined to be stunned.)

LOL. Funniest thing I've read all day.

 

[divegeek]

Federal Hydrashox. Best personal defense ammo on the market. Enough said.

What makes it the best?

 

[divegeek]

me = :noob:

What makes a smaller load better? This defies logic IMO. Please elaborate!

If you're looking at two rounds in the same caliber but with different bullet weights, there are so many factors that will affect relative performance that it's hard to make any kind of generalizations.

I won't let that stop me.

All else equal, lighter bullets will have higher muzzle velocities and will typically carry greater kinetic energy (not sure why they do; from the physics of the situation I would think that heavier slugs should pick up more energy during the traverse down the short barrel, but the measured numbers say otherwise). Greater kinetic energy may, in some circumstances, provide greater hydrostatic shock, particularly since the lighter bullets tend to shed their energy faster in the target. Greater velocity and kinetic energy may also improve expansion.

On the other hand, lighter bullets often have lower momentum, which results in a little less penetration. Of course all of the quality self-defense ammo for 9mm and up has decent penetration, but in some circumstances it might make a difference. Also, heavier bullets are harder to deflect which may make a difference, sometimes.

So which is better? Depends on what's in the bullet's path. How much difference can it make? Probably not much. It seems likely that for unobstructed center-mass shots, a faster, more energetic bullet may be slightly better than a slower, less energetic bullet in the same caliber, but the difference is going to be small.

Personally, I'm carrying 135 gr Gold Dot's in my XD9 because I got a good price on them at the gun show.

 

[divegeek]

Let me start this by saying IANAP (I am not a physicist), but I do play one on TV! The equation for kinetic energy is (.5)*(mass)*(velocity^2), so while a reduced mass will reduce the kinetic energy some, increased velocity has a greater effect (being squared) and can increase the net kinetic energy. A less-massive bullet would also, in theory, be easier to spin via rifling in the barrel leading to less energy lost in this process as well.

In general, if you apply the same force over the same distance to two objects with different masses (ignoring friction), their resulting kinetic energy will be identical, but their velocities will be different. So, if the expanding gases pushing the bullet provided a nice, constant force, and friction down the barrel were the same, the light and heavy bullets would have the same kinetic energy.

The faster bullet would also be spinning faster, which means that a greater portion of the energy is invested in rotational kinetic energy, which isn't measured by a chronometer, since it only sees forward velocity, not rotation. That should tend to make the lighter, faster bullet appear to carry less kinetic energy, not more.

However, you pointed out something I hadn't considered. Assuming they're made of the same materials, and since they're obviously the same diameter, the heavier slug must be longer than the lighter one, which increases the surface area to engage the rifling and increases friction. That friction scrubs off some kinetic energy and converts it to heat in the barrel, and a longer slug will lose more.

I don't know that the increased friction loss is enough to explain the energy difference, though. I think instead it's because the force from the expanding gases isn't a uniform, constant push down the length of the barrel. It's a fluid dynamics problem, and there are all kinds of complexities in the way the gas and bullet interact. Something in there, probably having to do with the fact that the heavier slug spends more time traversing the barrel. Maybe the gas pressure falls off more before the slug gets to the end of the barrel? Dunno. Barrel length also plays a role here, as does any loss of gas pressure from the chamber, leakage around the slug, the recoil action of the gun, etc. There are lots of factors and you'd need to be a better physicist than me to know what makes the difference.

 

[divegeek]

I think he was trying to point out that the inertial mass would take less energy to put in to motion along the rotational axises which is probably more than offset by the increased rotational velocity.

Doh! You're right that I ignored the fact that a lighter mass will take less energy to get up to speed. Not sure how I missed that one

You're also right that the effect would probably be offset by the increased rotational velocity. I'd have to think about how to calculate which effect would dominate; it's not clear to me.

The semantics of internal ballistics concern me far less than my group size

No argument here.

 

[divegeek]

Here is how important I consider the wight of a handgun projectile I carry a Kahr P9 originally bought for my wife where recoil was more of a concern. I wouldn't spend the money to switch to a .40 in the same platform. I have gone from Double Tap 115+p to federals 124 grain HST reason? I no longer trust my old ammo after finding a 147grain bullet in the 115s! Speer's 124grain short barrel load, corbon's loads and the rest of the premium ammo is just to expensive to shoot and rotate 19.00 for 50 rounds of the federal lets me establish reliability at a reasonable cost. Sure the super tactical octopus fanged super shredder load from Joe's tactical ammo has impressive advertising and can shoot the crud out of jello but, at close to or over $1 a bullet I cant afford to shoot enough to test the ammo gun compatibility. Unless there's some exploding uranium ammo its still a handgun! don't expect "one shot stops" unicorns, gnomes, and other fairy tale's to come true.

Hey, we're discussing an interesting physics problem here.

Stop trying to hijack our thread!

:lol2:

 

[divegeek]

Sorry, Swillden it been so long I am starting over with algebra again I just dont have the edjumication or intelect for that fancy college-boy stuff.

:lol:

I'm actually starting over with Algebra again, too... teaching my kids. Doing Algebra and Geometry this year.