Basic physics: Why do lighter bullets have exponentially more energy?

[DaleA]

The following may or may not be relevant.

I recently had a question (legal, not firearms related) that 'no one could answer' (in my later life I think I should trademark that phrase).

Knowledgeable folk tried to answer my question but they just didn't REALLY understand what I was asking. Everything was polite and there were no real hard feelings, but I wasn't satisfied.

Then a couple of days later, while I was working on some fiddly workbench stuff I realized what the knowledgeable folk had been trying to tell me and how it really did answer my question.

Long story short, I had a blind spot that prevented me from understanding. I don't know why it was there and I don't know why it went away but it left me feeling awfully sheepish.

[LeverGunFan]

Originally Posted by 44 AMP

.... I disagree. I just don't see things that way, absent more information relating to the specific loads. Here's my viewpoint, and, its going to contain that dreaded word "if".

IF the powder space in the case is the same (and it may not be, but IF it is) how can the same amount of powder, in the same amount of space produce different pressure??

COL is not an accurate representation of the amount of powder space. I have a Hornady book showing 2 different 185gr and 2 different 230gr bullets and the COL for each one is different. Hornady might be seating these bullets so the depth of the bullet base in the case is the same (which would make the available powder space the same) but there is no information given saying that is what they do. I guess it is implied, but not stated?? ....

Let's look at the data in the Hornady manual, mine is the 9th edition. There are four powders in common for the 185 grain bullet at 1000 fps and the 230 grain bullet at 850 fps. Here are the powder charges for the 185 gr @ 1000 fps vs the 230 gr @ 850 fps:

Power Pistol: 8.2 gr vs 6.7 gr
Longshot: 8.7 gr vs 7.1 gr
4756: 8.8 gr vs 7.3 gr
HS-6: 10.1 gr vs 8.2 gr

That's a significant difference (over 20%) in powder charges between the 185 gr and the 230 gr. Unfortunately Hornady does not list pressures, so I used Quickload for a few predictions for XTP bullets and Power Pistol powder.

Quickload agreed closely with the Hornady velocity data; Quickload predicted the 230 gr at 16.8k psi max and the 185 at 17.4k psi max, reasonably close. The useable case volume was not equal, the 230 XTP at COL of 1.21" had 12 gr water vs the 185 XTP at COL 1.213" with 14 gr water.

Adjusting the COL for the 185 gr bullet to get the same useable powder volume (1.156" vs 1.213") and using the same 6.7 gr powder charge as the 230 gr, the predicted pressure is 14.3k psi, 2.5k psi less than the 230 gr. The 185 gr bullet is easier to move down the bore so less max pressure at the same powder charge and useable case volume. EDIT: To get to 17k psi max at this COL in Quickload, the powder charge was increased from 6.7 gr to 7.3 gr.

Adjusting the COL even lower to get the same max pressure (1.12" vs 1.213") gave a predicted pressure of 16.8k psi, but with a COL that may not feed reliably.

So I stand by my statement that all else being equal (COL in the same range, as it is in the Hornady manual), the 185 gr will require a larger powder charge to get to the same max pressure as the 230 gr.

[Aguila Blanca]

Originally Posted by 44 AMP

First, I think that the example he is using is not about powder charge, and certainly not about what specific powder do in real world cartridges. I think what he is asking about is bullets launched with equal force, and how their energy numbers are different, and why.

IF different weight bullets were launched with a spring (mechanical energy) and that energy were constant, his question would still be the same. So I think focus on the powder charge isn't answering the question.

Well, he specifically said the same powder charge, but you may have a point about that not being possible. Your comment reminded me of an article I read on the Shooting Times web site several years ago. I copied it and saved it for the simple reason that the examples were based on .45 ACP (which is what I shoot, Winchester 231 (which is what I use for most of my handgun reloading), and two of the three bullets discussed (Berry's 230-gr round-nose and Berry's 185-gr hollow-base round-nose) are what I load.

Here's the article: https://www.shootingtimes.com/editor...ves-edge/99399

For some reason, the images aren't showing up for me now, so I have to refer to the copy I made to see that charts and tables. What the article is about is making a 165 power factor with different bullet weights.

What the author found was that with a 230-gr bullet to make 165 power factor he needed 5.2 grains of powder, but to make the same power factor with the 185-gr bullet he needed 6.6 grains.

Viewed another way, he also included a chart showing the resulting velocities and power factors for representative powder charges. What that table shows is that FOR THE SAME POWDER CHARGE lighter bullets do NOT achieve higher velocities -- they are slower.



This supports my own findings that I typically needed more powder to drive a lighter bullet to the same velocity (not muzzle energy) as a heavier bullet.

[Mustey]

Sorry for not posting again.
I am definitely still interested in this.

Thing is, actually...
Last time I loaded my own was 10 years ago. I can't even remember the brands of components used. I only remember I used the same amount of powder.

At the time, I didn't care. I "knew" lighter bullets come out faster and get more muzzle energy.
Same way I knew a 3GHz processor is faster than a 2GHz processor - I didn't stop to think "why" until I studied electronics engineering.

I'll read carefully all the comments over the weekend.

I can't replicate and demonstrate the problem with a firearm, as I currently live abroad.

How about a spring air gun?
The piston is very consistently loaded to the same energy state.
And it has the added benefit that airgun pellets have very similar friction profile, no matter their weight - they all seal at the skirt, which gets blown up by the pressure and expands to the diameter of the barrel - but the contact area must be pretty much the same (very small) - so, it's only going to be the weight that significantly changes...
Sunday I'll have access to a chronograph at the local range

[Mustey]

Just skimming the answers I see I could have totally got it wrong and lived in a lie for decades

Now I am not 100% sure the sun will rise tomorrow

I guess while the discussion wasn't progressing humanity and helping everyone involved transcend to a space fairing civilization, it did fix a misconception I had and it made a difference to ONE person

Thanks for all your lovingly prepared answers

[44 AMP]

Yes, the OP did say "the same powder charge" and he also said "same amount of chemical energy, and he also said this,

A) I load a 45acp with a 230-grain bullet and it does 850fps = 370 ft-lb of energy
B) I load the same shell with the SAME powder charge with a 185-grain and it does 1000fps = 411 ft-lb of energy

Not mentioning anything about what powder or charge was involved in creating that data.

It may be we are chasing our tails, arguing about how many angels can dance on a pin trying to explain what may be the OP's flawed understanding, or perhaps just use of terms in a way I'm sure he understands but leave doubts in the rest of us.

As for the data in a Hornady manual, I'm sure they accurately report the results they got with what they tested. One manual gives one set of data and another gives another, and slightly different set of data. I've got manuals that list powder charges a couple tenths off (Both higher and lower) than yours does.

Your gun is not their test gun. My gun is not their test gun, and it won't be the OP's gun, if he has one, and absolutely identical results across the board I would put down to serendipty or simply random chance. The manuals are guidelines, not holy writ and while one expects similar data no manual should be considered unalterable dogma.

Also, considering that Hornady lists the velocity in blocks of 50fps, one cannot and should not expect too much precision. And, its not needed.

[tangolima]

I bet heavier pellet will go slower but yet will give you more energy.

-TL

Sent from my SM-N960U using Tapatalk

[rickyrick]

I’ll take a light fast bullets over heavy ones for any given rifle caliber/cartridge if available; this is for anything intended for mammals and/or large aggressive reptiles or fish that may have come on land to challenge me lol. Don’t really care when it’s plinking.

Just my opinion based on my own experience, higher velocity bullets work better for me.

[jackstrawIII]

The easy answer is energy transfer. The lighter bullet has less inertia, therefore it is able to receive more energy from the powder in the short amount of before it leaves the barrel.

The downside is that the lighter bullet also loses energy quicker because of the lower inertia. This is why long range hunters/shooters prefer a heavier bullet, which retains energy better at longer range by losing less energy to drag. Obviously ballistic coefficient plays in, but the principle is true even without factoring in BC.

This is not complicated.