Impact!

[CharlieNC]

Does the performance of a bullet upon impact simply boil down to kinetic energy, KE = 1/2 MV^2 as typically touted by the high velocity advocates? My friends who run a long range steel shooting facility observe this is not always the real world result, and it is often easier to spot impacts on steel or misses on dirt with the slower, heavier bullets.

For fun I decided to compare a couple of cases. Being so long since looking at the calculations I googled "ballistic pendulum" for a refresher, and you will find many reports and videos on the topic; fortunately they are all very consistent as this is an age old study. A few primary points:

- The impact of a bullet on an object is an inelastic collision. In this case the laws of conservation of momentum, P=Mass*Velocity apply. While there is conservation of energy, there is NOT conservation of kinetic energy. Energy is the ability to do work and can take many other forms rather than KE.
- The ballistic pendulum is a block hanging on a string, and when impacted by a bullet it swings upward in an arc. The height,h, to which it is lifted upon swinging represents the conversion of the kinetic energy imparted by the bullet, into potential energy due to the gravitational effect on the target mass as PE = Mgh. There is a significant loss of kinetic energy during this transition into potential energy.

Example
Shoot a 10lb steel plate target at 1000 yards. Consider a 178 gr 308 impacting at 1045fps vs a 105gr 6mm impacting at 1500fps.
- Impact KE: 308 @ 432ft-lb 6mm @525ft-lb
308 @ 586 Joules 6mm @712 Joules

- Impact Momentum: 308 @ 3.68 kg-m 6mm@ 3.11 kg-m

So the 6mm hits with more kinetic energy, but the 308 with more momentum. What about the effect on the target, ie how does the steel plate on the hook behave?
- Target velocity: 308 @ 32 inches/sec 6mm@ 27 inches/sec
- Target height: 308@ 1.3 inches 6mm@ 0.9 inches

Thus the 308 will move the steel plate faster and higher upon impact due to its higher momentum vs the faster 6mm which has more kinetic energy. Interestingly in this case over 99% of the kinetic energy is lost during the impact!

So what? If you are shooting steel targets and need to see impacts better, meaning you can spot misses easier and have less risk of the spotter not seeing the impact on the steel plate then momentum vs kinetic energy is the answer. This evidently goes against most of what you have seen advertised.

What about hunting? Bullet performance ( expansion, fragmentation, etc) plays a dominant role and is dependent on impact velocity. This is much more complex than these simple formulas can begin to approach, meaning it is also much more than simply assuming the highest KE round is superior as inferred by many manufacturers.

This is insight from a novice and meant to be a seed for discussion vs a definitive answer, and I would be interested in insight from others who have considered this more so than I.

 

[PigButtons]

Well done real world explanation. Thanks. I do not have anything useful to contribute however.
But I think the people shooting Bullseye would be very interested.

 

[Old Navy]

Thus the 308 will move the steel plate faster and higher upon impact due to its higher momentum. This is what I have observed the few times I shot my 6br and 6.5X47 the same day. Bigger splat on the target also.

 

[7887mm08]

Sorry but this is a pay grade above me, so i will take your word for it!

 

[6ShotsOr5?]

I don’t think people generally select lighter, faster bullets for the energy. For things like 400-600 yd varmint/predator hunting, it’s so they can shoot flatter and have a more forgiving situation when they are off by 10-20 yds on the range to target. Even though they start slower, heavier bullets with higher BCs lose velocity slower and may deliver more KE downrange than a lighter, higher MV bullet.

 

[Doug Beach]

It all depends on what you want from the impact. On a steel plate, a well thrown fastball, and .45 hardball would give you approximately the same swing, from similar momentum. Both could also kill you, but one is a LOT more likely to.
That fastball would would have to double its speed, in order to have the hardball’s energy. At which point I suspect it would be approximately as lethal.

 

[std7mag]

Seeing splash is more about mass than velocity.
Which leaves the larger impact in dirt. A thrown pebble, or a brick?

 

[Tokay444]

There is no replacement for following your bullet, in your scope, all the way to impact. Try that with a 19# rifle shooting a 178gr 308, and again with 105gr 6mm in the same 19# rifle.
It becomes more about recoil management than target momentum.

 

[BillJ]

Flatter is better, IMO.. Unless you are speaking about a woman's... ??? heels...

 

[Trophied]

I feel so dumb, but strangely I do understand the conclusion! If only I'd had an Algebra teacher as competent I may have understood it.

 

[timeout]

This doesn't have anything to do with anything probably but - I'll always remember shooting a steel antifreeze can full of water with two very different caliber rifles. The first one was a 30-30 lever action. It punched a clean and neat 30 cal hole through the can. Next up was a .243 bolt gun with some 70 grain Hornadys. Fresh new antifreeze can full of water. Blew every seam out of that antifreeze can in an impressive display of water exploding every which direction. As a young 20 year old I was impressed by what speed does.

 

[CharlieNC]

My motivation for looking at this was my range operator/shooting buddy has gone full circle away from the fast, superior high bc 6mm back to 6.5 and 308 in order to achieve higher PRS scores. Spotting misses and spotters not recognizing long range steel hits were the culprits.

 

[dskogman]

Here’s my take on it. It takes energy to move the plate. The ballistic pendulum references use both conservation of energy and momentum to calculate bullet velocity And height the plate moved not one or the other. They also try to capture as much energy as possible by shooting into a wooden block or sand filled cylinder. http://hyperphysics.phy-astr.gsu.edu/hbase/balpen.html
Shooting a steel plate is different in that it converts some of the energy into moving the plate but most of gets converted into heat and lateral movement of the bullet fragments aka splash. A small high velocity bullet is going to lose more energy in the impact than a slower bigger bullet. They almost get vaporized in the impact. A bigger bullet has more lead moving at slower velocities in the splash so it’s easier to see.

 

[rr2030]

I understand I am somewhat off on a tangent here. 40 years ago, when IHMSA was a big thing, studies were done one a bullets ability to knock down a properly set regulation 60 lb Ram at 200 meters. Much to do with caliber, bullet weight and shape as much as pure terminal energy. Had much to do with how long the bullet hung on the steel transferring its energy to the steel.

I would love to elaborate, but I can't locate the documentation, and the brain cells that contained much of the then necessary information, fell victim to an assault by a bottle of single malt Scotch back in the late 90's.

 

[miningshawn]

Here's my take.

After slogging through Physics 1, 2, and 3 - Statics - Dynamics - Strengths of Materials and more math than I could ever use....

My .308 (185gr at 2620) spins the "spinner" target at PRS matches in 3 rounds, every time. The 6.5 (140ish at 2880ish) guys almost always use five. The "hot" 6's are lucky to spin it with 5.

I started getting interested when shooting silhouette next to the BCPR guys. There's a reason the minimum caliber is 6.5, and I've seen the rams laugh off 6.5 Creedmoor on quite a few occasions.

'Nuff said.

 

[vinconco]

Factor in the reactivity and angle of the target too

 

[DirtySteve]

You should see what happens when a 500 grain bullet hits steel from a 45-70 at 200 yards when compared to any modern cartridge. It is like shooting a VW at something. Once you get that big slug going it real doesn’t get slowed down by much.

 

[std7mag]

I started getting interested when shooting silhouette next to the BCPR guys. There's a reason the minimum caliber is 6.5, and I've seen the rams laugh off 6.5 Creedmoor on quite a few occasions.

'Nuff said.

Careful!
You'll get canceled faster than a Trump tweet about Quid Pro Joe's kid by picking on the mighty Manbun!

You should see what happens when a 500 grain bullet hits steel from a 45-70 at 200 yards when compared to any modern cartridge. It is like shooting a VW at something. Once you get that big slug going it real doesn’t get slowed down by much.

Except air resistance. BC of a barn door.
Weight of the barn.
People still don't realize that their car doesn't stop a train.