Different powder charges = different SDs?

[ChristianPatriot]

Can somebody explain that to a noob? How is it that some powder weights have smaller deviations than other weights? Those two things seem unrelated. All this is in the context of load development for a long range rifle.

 

[45fan]

Guess I am not picking up what you are putting down. Are you asking why some powders have less deviation between min and max load weights?

 

[Mgderf]

SD= Sectional Density

Powder charge has nothing to do with the physical characteristics of a bullet.

 

[ChristianPatriot]

I'll try it this way. A guy is shooting 5 shot groups of different powder charge weights to figure out what his rifle likes. Same powder, different charge weights, velocity deviations. Let's 43.2 grains groups really well but has a 20fps spread in velocity among the 5 shots. 44.6 grains groups ok, but only has a 5fps spread in velocity among the 5 shots. This is not my personal experience but one that I've read many times. Has anybody experienced this?

 

[ChristianPatriot]

SD = Standard Deviation.

 

[Seancass]

There's approximately 1.5 million things that play into this. General rule of thumb would be low SD is good, but accuracy has the final say.

Why does the SD vary? Lot's of reasons! First off: Statistics. Shoot 20 shots and then talk about SD. Although I've done this myself, 5 shots isn't really significant. Again, accuracy is more telling here.
Another reason could be how you seated and crimped the bullet, was it absolutely perfectly uniform.

How can a load be more accurate AND have a larger SD? Well, that goes back to there being 1.5 million contributors to accuracy. 20fps on your 3000fps load is less than 1%. Did your twitchy little finger squeeze within 1% consistency? What about your thumping heart or that little breeze? Did your bullets weigh within 1% of each other? How perfect was your powder charge? SD is just one small contributor to accuracy.

Why does one load produce a different SD than a similar load with only powder charge changing? Well, all powders have a happy place. Get the right fire lit behind the right bullet in the right barrel and that rifle will just sing a song. Change just a little and you have the wrong fire lit. Do you speak enough GearHead to know about air-fuel mixtures? It's a pretty fair analogy. Adding more fuel doesn't always make the motor happier! There's a sweet spot and it can vary from motor to motor and even day to day.

I hope this rambling post provided some food for thought, if not an actual answer.

 

[Leo]

Simplistic explanation. A physics professor would need 300 pages to express this, but would include definitions and equations.

Pressure is part of the burn equation. Too low of pressure, weak, unsteady burn, variance in acceleration.
The right pressure curve makes a smooth consistant burn that can readily be applied to accelerate the projectile uniformly.
Too high or too fast of pressure build curve makes a less consistant acceleration, raising the standard deviation variance.

 

[Broom_jm]

Simplistic explanation. A physics professor would need 300 pages to express this, but would include definitions and equations.

Pressure is part of the burn equation. Too low of pressure, weak, unsteady burn, variance in acceleration.
The right pressure curve makes a smooth consistant burn that can readily be applied to accelerate the projectile uniformly.
Too high or too fast of pressure build curve makes a less consistant acceleration, raising the standard deviation variance.

I'd REP you again for this comment, but apparently I'm supposed to spread the love.

@ the OP...Don't obsess about why one charge had a higher or lower Standard Deviation. Also, unless a load with a low SD also shows great accuracy, don't pay a whole lot of attention to SD at all. The proof is in the bugholes.

 

[45fan]

Simplistic explanation. A physics professor would need 300 pages to express this, but would include definitions and equations.

Pressure is part of the burn equation. Too low of pressure, weak, unsteady burn, variance in acceleration.
The right pressure curve makes a smooth consistant burn that can readily be applied to accelerate the projectile uniformly.
Too high or too fast of pressure build curve makes a less consistant acceleration, raising the standard deviation variance.

Adding to your explanation, the harmonics, and the way they effect the specific barrel the load is being shot in make a big difference in accuracy. This is why a load that bugholes in one rifle will only pattern in another. Its also why the Browning BOSS system worked as well as it did. Tuning the harmonics of a specific load into a barrel by moving weight up and down the barrel in effect does the same as changing the charge weight slightly up or down.

 

[ChristianPatriot]

I appreciate the education. It all makes sense.