statjunk

Hey Guys,

So for years now it always comes up .270 better than this or that. etc....

When deciding what is better than another where does bullet weight come into play?

Said another way, how can we compare a .270 to a .308 if they shoot completely different bullets?

Should retained energy be the measurement?

On another level I'm wondering since there appears to be lots of factors, kick, bullet weight, amount of powder, trajectory etc... if there couldn't be a mathematical model that quantifies a cartridge. Basically every cartridge gets a number based on it's factors. Does this make any sense?

Ok I'm done rambling let me have it.

Tom

dmurphy317

I'm not sure if its what your talking about but there is a comparison tool called the Taylor KO factor.So in your comparison of a 270 and 308 if they both are shooting 150gr bullets,with the 270 at 2850fps the TKO is 16.92 and the 308 at 2936fps the TKO is 19.38. Load data taken from the IMR loading data online. I'm not sure what the numbers give you other than a basis for comparison based on the bullet weight, velocity and caliber.

ShatoDavis

what you are asking for already exists. But, like most things its not nearly as simple as we would like them to be.

How do we compare cartridges with different bullets? You must use sectional density. To properly compare two cartridges of different diameter bullets you must compare bullets of similar SD. for example a .270 140gr. bullet has a SD of aprox. .26 to compare a .30 150gr. has a SD of aprox. .26

Now you can compare the two.With this in mind you can Look at the muzzle velocities, kinetic energies, trajectories...... Recoil is so subjective I don't have any clue as to how to quantify it. I know that there are measurement in foot pounds etc. But what is manageable to one person can be unbearable to another.

I will tell you this, you won't find a nickels worth of difference between them anywhere but on paper. In the real world of average shooters at average distances game animals are just as dead with one as the other. The rest of the arguments are just bored hunters trying to fill their time until the next hunt begins.

SwampCollie

Good post. Especially the last paragraph.

James B

Shato said it just right. SD is the way to compare bullets of different calibers to each other There are to many other differences to make this the one and only method but at ranges to 300 yards there are a truck load of cartridges that will perform the same task equally well. One other component is to compare bullets of the same shape and construction. Such as using the Partition in all calibers for comparison.

ipscshooter

Remington and Winchester both show their 150 gr. .270 loads at 2850 fps, and their 150 gr. .308 loads at 2820 fps. He said he was using IMR's reload data, so I suspect there's may be a pretty wide range of combinations for each rifle... Perhaps the .308 load he was looking at was a "light magnum" load, vs. a standard .270 load.

statjunk

So how can a .270 be compared to a .308 when a .308 can launch heavier bullets but a .270 cannot?

For the record I know this is just a bunch of boo haa because they all work inside of 300 yds. Just trying to figure out if it even makes sense to make a comparision in the first place.

Tom

James B

You compare the two by using the 150 grain 270 and the 180 grain 308. You can expect these two bullets to perform about the same job because they have the same SD. Both will penetrate about the same and in this case the velocities and ME are in the same ball park.

The taylor KO fomula will give you a good tool to compare even though its not perfect and leans toward Heavier bullets.

BrutalAttack

What about ballistic coefficient and drag coefficient?

The "closeness" of a bullet to the mathematically "perfect" bullet has to play into bullet efficiency and thus cartridge efficiency at some point. See Krupps Standard Reference Projectile.

There are some basic models out there and I believe they were developed by the Army's Ballistics Research Lab. You've probably seen them in small ballistic programs usually they are called something like G1, G5, G7, etcdrag coeffiecient models.

The short comings of those early models gave rise to the BC or ballistic coefficient, bullet form factorand the sectional density models which helped relate different bullet shapes and sizes to the drag models. Which is why we generally use Ballistic Coefficient as a measure of how much "better" a bullet is moving through the air.

So SD is not the product we want to look at. It's a component of BC which is what we currently use to compare bullets.

Ballistic Coefficient = (Bullet Sectional Density) / (Bullet Form Factor)

I'm sure computational fluid dynamics has been applied to conventional rifle cartridges and there is a model and I'm sure it uses a number of variables to spit out a betterproduct but the question remains: how do you define "better"? Also, what assumptions is your model based upon? The answers, if there are any, have an enormous effect on how and what your model shows, and if those findings are worth anything in the real world.

Also, a complex fluid dynamics model that approached anything relative to real world performance may not even be able to be ran on anything short of a mini-supercomputer. And certainly not a normal PC.

Now that all applies to exterior ballistics.

If you're talking about terminal ballistic performance, no I don't believe there is any model for terminal ballistics. At least one that would produce anything worthwhile or close to reality. The inherent stochasticityand sheer volume of variables of such a model would be impossible to simulate in my opinion.

James B

There would be no end to the things that complete the whole picture. The question here was just how to compare bullets of different wights and caliber. To compare you would need to use the same bullet as well. Like the Nosler partition which has the same shape and construction in all caliber. I am a hunter not a scientist or ballistics engineer. I have however killed big game in the field with probably 20 different calibers. I don't know everything about why things work but I do know what does work and what does not.