hypothetical question regarding powder choice

[Teletiger7]

I develop 2 loads for .308 using the exact same components(same brass, primers, bullets, OAL, etc.)except for the powder. One powder is slower burning and the other faster burning. I adjust the powdercharge for each load so they both will shoot the bullet out at the same velocity. Question: Should both loads shoot just as accurately? Or will the different powder burn rates cause a difference in accuracy?

[762cavalier]

Yes different powders can cause differing accuracy. Even though velocity is the same, the pressure curve is different. This can cause accuracy differences. Each rifle is different and one rifle may shoot accurately with the first powder and a second rifle may like the second powder more. that is why load development is crucial.

[swerv512]

i'm no expert, but burn rate should cause a difference in accuracy. depending on barrel length and twist(?), your rifle may like either or the two. try them out and see.

[GrayWolf09]

For reasons I cannot understand, some rifles prefer some powders and bullet weights more than others. My Savage 110 in .243 loves Sierra 85 grain HPBT and 4064 powder. Other bullets and powder not so much. Half the fun of reloading is finding the right combination. I do not understand women either, but both are fun!

[Fjold]

There is a difference in when peak pressures are reached which will effect accuracy. Think of it as throwing a bowling ball in the conventional manner and hitting one with a BF hammer. The velocity may be the same at the release point but the way that the force is applied will change stuff like barrel harmonics and the recoil levels, etc.

[Teletiger7]

Thanks for the input everyone.

[buffybuster]

I develop 2 loads for .308 using the exact same components(same brass, primers, bullets, OAL, etc.)except for the powder. One powder is slower burning and the other faster burning. I adjust the powdercharge for each load so they both will shoot the bullet out at the same velocity. Question: Should both loads shoot just as accurately? Or will the different powder burn rates cause a difference in accuracy?

As others have stated, the powder choice can have an effect on load accuracy. Both loads CAN be equally accurate?: Yes. Will they be equally accurate?: Too many variables to determine.

Pressure curves for each powder are going to be slightly different. Also the rifle and ammunition is a "system". That system is like a large tuning fork. During the instant of firing the entire barreled action is vibrating (harmonics). Exactly where in the harmonic wave the bullet exits the barrel is going to have an effect on accuracy. These accuracy nodes that people discuss is about finding when the bullet exits, either at the top or the bottom of the wave. Those points will provide the most consistent release points. The Browning Boss and these rubber gromments are methods to externally tune these harmonics. Free floating the barrel is to allow the barrel to vibrate normally.

Load density and flame propagation will have an impact on the internal ballistics. As a general rule, the load with higher load density will tend to shoot better than lower density loads.

[jwest]

As buffybuster said - theoretically - the load with the load density closest to 100% or over (compressed load) will theoretically perform better. Has to do with the powder abutting the bullet which pushes the bullet out in a more uniform fashion - also theoretically results in tighter grouping.
Of course - a lot of other factors apply - perhaps a study in ballistics will help - Hornady has some good stuff on their site - plus there are a plethora of books out there on this topic.

[ar15barrels]

It's all about harmonics.
Quickload has a feature that lets you find loads based on time of bullet in the barrel.

[buffybuster]

The barrel will vibrate in a sine wave of 2pi duration over it's unsupported length. Various tuners and bedding methods are an attempt to adjust this unsupported length to adjust the frequency of the sine wave. By adjusting the frequency you can time the bullet to exit at either the top or bottom of the wave. Those are the two points a which the barrel is not moving, which should provide the most accuracy.

This is the primary reason there is a POI shift when you put a brake or suppressor on the barrel.

[jwest]

Randall,

Took a look at the Quickload demo - pretty nice.

Here's an interesting page on barrel harmonics:

http://www.varmintal.com/atune.htm