Share your immense knowledge on powder behavior

[Mordamer]

Here is the question: If you have two powders of similar burn rate, say Varget and A2520, and you are loading them to create the same pressure signs/muzzle velocity, can the two powders create drastically different accuracy from a certain gun?

In other words, if you keep all other loading variables constant and you only change the powder type (and say that you keep the chamber pressure the same), then could you get accuracy changes of 1/2 to 1 MOA?

I'm wondering if anyone knows a lot about powder and maybe some other quantified characteristics besides burn rate.

 

[MitchAlsup]

Re: Share your immense knowledge on powder behavior

OBT (optimal barrel time) theory indicates that if the two bullets take the same amount of time to traverse the barrel, they will fly in the same direction due to barrel harmonics.

So, given the same gun, and the same barrel time, you question is afirmed. So the question then becomes: if similar burn rate powders are loadsed such that the pressure peak occurs at the same time and has the same peak pressures, and the muzzle velocity is the same: it is going to be difficult to find something inthe firing process taht would lead to greater or lesser accuracy.

However, some powders coat the barrel internals differently and these can lead to different accelerations of even the same bullets. One can see this effect in a pair of loads that are both accurate in a given gun when shot all by themselves, but are not as accurate for a few shots as one barrel fouling chemistry gives way to a differen barel fouling chemistry. After a few shots, the barrel comes back around and is good again.

{Yes, I know I did not quite answer your question.}

 

[Mordamer]

Re: Share your immense knowledge on powder behavior

You did a pretty good job at throwing out your knowledge. When I pose a theoretical question on here I usually know what the general answer is, but I just like to see what the guys will say. I appreciate you answering and giving it a shot while it seems many others have not tried.

 

[Mordamer]

Re: Share your immense knowledge on powder behavior

Maybe this will give people a better idea of why I posed the question.

I'm using varget to launch 75 grain Amax bullets. I'm using pretty equivalent loads of A2520 behind the same bullets at the same seating depth. The powder burn rates are not that far off. The pressure signs are similar. I keep everything constant as best I can except for the powder type, but I get about 1.5" groups with the A2520 and about .5 or better groups with the varget. I can't figure it out.

 

[Austan]

Re: Share your immense knowledge on powder behavior

What is the velocity difference between the 2 loads?

 

[Mordamer]

Re: Share your immense knowledge on powder behavior

I don't know the exact velocity at the muzzle. I'm guessing it would be within at least 100 fps of each other. Maybe even closer. I'm hoping to get a chrono for christmas.

 

[Fuzzball]

Re: Share your immense knowledge on powder behavior

Bottom line, we can't get the same time/pressure/velocity response from different powders. If we did, they would be the same powder.

 

[bohem]

Re: Share your immense knowledge on powder behavior

On the topic of Optimal Barrel Time there's a piece missing. The shockwave propagation is important. You can get very similar bullet transit times but if the shock wave is in a different place when the bullet exits you have different repeatability characteristics.

The shock wave is going to move the same speed in the same barrel, it's dependent upon the material and material shape, not the powder burn rate. The pressure curve will help dictate the bullet transit time.

I think I remember the shock speed being around 6km/s in steel. Which means that the shock wave bounces back and forth between the crown and the breech about 20 times before the bullet exits.

If the bullet exits the barrel 5% sooner or later with A2520 than it does with Varget you get different results.

It's very possible that your Varget load is leaving the barrel with the bullet near the breech and the A2520 load leaving when the shock is near or at the muzzle, which would create poor repeatability and/or oddball fliers

 

[MitchAlsup]

Re: Share your immense knowledge on powder behavior

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mordamer</div><div class="ubbcode-body">I'm using varget to launch 75 grain Amax bullets. I'm using pretty equivalent loads of A2520 behind the same bullets at the same seating depth. The powder burn rates are not that far off. The pressure signs are similar. I keep everything constant as best I can except for the powder type, but I get about 1.5" groups with the A2520 and about .5 or better groups with the varget. I can't figure it out.
</div></div>

You are comparing only the burn rate. Varget is a single base stick powder while 2520 is a double base ball powder. So while the burn rates are similar, they are enough different that a different load recipie is needed.

Thus, start low, and run the three fold dial in.
a) find the point of pressure
b) find a CW just under pressure that has good group size
c) find the width of the charge weight window where group size remains good.
And set the thrower to the mid point of this window.

 

[Greg Langelius *]

Re: Share your immense knowledge on powder behavior

Hot /cold barrel... Does this make a difference?

 

[Mordamer]

Re: Share your immense knowledge on powder behavior

Well, I can get varget to shoot excellent groups. I have tried different load values of the A2520 all the way up to pressure signs and can't seem to get it to shoot well anywhere. I was surprised by these results. I had a few pounds of the A2520 and I wanted to see if I could get it to shoot as good as varget. I think I'll put it back on the shelf and buy some more varget.

 

[gunsnjeeps]

Re: Share your immense knowledge on powder behavior

I think that this may have been explained differently than I am saying it. Say powder A gives a high spike and pushes the bullet in to the barrel really fast, powder B is ignites slower but has has the same pressure spike later in the burn. The barrel will vibrate differently and the muzzle will be in a different place when the bullet leaves it. Looking at a burn rate chart (there are several, and a few differences in each) and reloading tables, 2520 is a little slower and has a higher max load and speed than Varget.

 

[1917]

Re: Share your immense knowledge on powder behavior

From Recreational Software, Inc.

Powders with similar burn rates (quickness) as listed in a reloading manual often will not deliver similar results. There is a simple reason for this. Burn rate is only one powder characteristic and may not be the primary factor which contributes to accuracy in a particular caliber.

Composition and granule shape also affects the powder's performance and can vary between powders of similar burn rates. The burn rate of a powder is controlled by its base composition, granule size, granule shape and often a coating on the granules.

These design features also change the density of powder so that for a given weight there is more or less gas energy produced for a given volume that the powder occupies.

The percent of the available powder space under the bullet filled by the powder is called the "load density".

 

[MitchAlsup]

Re: Share your immense knowledge on powder behavior

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mordamer</div><div class="ubbcode-body">I have tried different load values of the A2520 all the way up to pressure signs and can't seem to get it to shoot well anywhere. </div></div>

What step size did you use between steps?
How low and how high did you survey?
how close to or far away from the lands are the bullets?
Did you give the barrel 10 rounds to restabilize with the different powder?