Burn rate changes with case volume ?

[big5ifty]

I was gathering information on relative burn rates, and I came across a statement on the Norma site :

"Ranking differences result from the use of different criteria. For example, a certain powder type can act slightly quicker than some other powder type, when used in 308 Winchester loads; however, when these powders are used in 7x64 loads, evident burning rates might reverse"

In this rifle of mine, I encountered that with IMR 4955 and Norma 204. IMR 4955 comes in lower on the burn rate chart than Norma 204, but in this rifle, for my load, the same quantities of those powders with the same bullet and primer gave higher pressure with IMR4955. I measured the expansion of virgin brass with a 1/10,000 micrometer, the physical expansion was greater with the IMR powder.

That's an interesting phenomenon to keep in mind when looking for equivalent burn rate powders - the published burn rate is particular to a case volume, which might not be the same as yours.

 

[RegionRat]

the published burn rate is particular to a case volume, which might not be the same as yours.

There are burn rate ranking charts, and there are parameters like the ones folks feed into programs like QL and GRT.
Most times, those published (caloric) burn rates are based on equipment that doesn't even resemble a case at all.

Powder companies do tests on standardized equipment to control their process, later on, other folks publish a ranking chart that often has no connection to the actual thermodynamic testing but instead is just based on anecdotal perspective.

So both ways, the actual powder caloric tests and cartridges don't really match up. QL and GRT values are really the result of a crowd source refinement process rather than hard data coming off of an instrument.

 

[mikecr]

Everything affects powder burn rate.

 

[AlNyhus]

Expansion ratio is the big determiner on on how powders act. -Al

 

[Webster]

I was gathering information on relative burn rates, and I came across a statement on the Norma site :

"Ranking differences result from the use of different criteria. For example, a certain powder type can act slightly quicker than some other powder type, when used in 308 Winchester loads; however, when these powders are used in 7x64 loads, evident burning rates might reverse"

In this rifle of mine, I encountered that with IMR 4955 and Norma 204. IMR 4955 comes in lower on the burn rate chart than Norma 204, but in this rifle, for my load, the same quantities of those powders with the same bullet and primer gave higher pressure with IMR4955. I measured the expansion of virgin brass with a 1/10,000 micrometer, the physical expansion was greater with the IMR powder.

That's an interesting phenomenon to keep in mind when looking for equivalent burn rate powders - the published burn rate is particular to a case volume, which might not be the same as yours.

Primers can change the burn rate. Some have a bigger charge and a bigger flame. A year or so ago someone put pictures up on this website of many different primers being fired in a dark room. Huge differences. Not only case volume but bore diameter and how fast the bullet is moving. The powder burns in the case and the barrel. Seems it would negate differences in case volume and wt.

 

[noylj]

Take True Blue
In some cartridges it is almost as fast as AA2. In another cartridge it is slower than Silhouette.
Always start at start load and work up. Never guess or assume.

 

[William Vogel]

I was gathering information on relative burn rates, and I came across a statement on the Norma site :

"Ranking differences result from the use of different criteria. For example, a certain powder type can act slightly quicker than some other powder type, when used in 308 Winchester loads; however, when these powders are used in 7x64 loads, evident burning rates might reverse"

In this rifle of mine, I encountered that with IMR 4955 and Norma 204. IMR 4955 comes in lower on the burn rate chart than Norma 204, but in this rifle, for my load, the same quantities of those powders with the same bullet and primer gave higher pressure with IMR4955. I measured the expansion of virgin brass with a 1/10,000 micrometer, the physical expansion was greater with the IMR powder.

That's an interesting phenomenon to keep in mind when looking for equivalent burn rate powders - the published burn rate is particular to a case volume, which might not be the same as yours.

You should download Gordon Reloading Tool and use it to create predictions for loads. You’ll find that using a burn rate chart is only useful for steering your powder choice. The burn rate chart alone will have you buying plenty of powder that’s not what you need for optimal results. I have a shelf full of different powders that I have to make work somewhere other than in my chambers. The software is free and while it’s not perfect, it’ll save you a lot of money and time finding accurate loads that are safe. It allows you to input case volumes, OAL etc and that alone will make you see that vast differences between your actual loads and what’s published in reloading manuals and the effects on powder efficiency. I got it and used it to close in on the most accurate and safe loads within a very short time vs aimlessly shooting bunches of components to be disappointed with results that questioned my ability to aim because of the group size. I have five out of six rifles that shoot less than 1/4 MOA now at 100 yards and less than 1/2 MOA at 700 yards because I used GRT to pick powder and charge weights that are the velocities and burnout needed for the performance I’m aiming for. It has a feature called Optimum Barrel Time that predicts accuracy nodes and I find it not to be exactly accurate but within the feature you can adjust the prediction of velocity to your actual chronograph velocity and it changes the pressure map which I have found to be fairly consistent with my fired brass condition and this should give you comfort in moving up powder charges. I’ve got two long freebore 223’s that shoot 75 and 80 grain bullets at 2980 and 2950 fps. Based on reloading manuals, I would be so overloaded, I’d never had chosen the charge without slowly increasing until pressure and always questioning the brass condition. I can go farther with the velocity at this point but with nearly 3000 fps I’ll probably never get another accuracy node and I’m satisfied. This recommendation is based on actual experience and I’d hate not to share it because it’s a free tool and it’s fun to explore. I love making things go bang but this will increase your confidence and make more shots count.

 

[Webster]

You should download Gordon Reloading Tool and use it to create predictions for loads. You’ll find that using a burn rate chart is only useful for steering your powder choice. The burn rate chart alone will have you buying plenty of powder that’s not what you need for optimal results. I have a shelf full of different powders that I have to make work somewhere other than in my chambers. The software is free and while it’s not perfect, it’ll save you a lot of money and time finding accurate loads that are safe. It allows you to input case volumes, OAL etc and that alone will make you see that vast differences between your actual loads and what’s published in reloading manuals and the effects on powder efficiency. I got it and used it to close in on the most accurate and safe loads within a very short time vs aimlessly shooting bunches of components to be disappointed with results that questioned my ability to aim because of the group size. I have five out of six rifles that shoot less than 1/4 MOA now at 100 yards and less than 1/2 MOA at 700 yards because I used GRT to pick powder and charge weights that are the velocities and burnout needed for the performance I’m aiming for. It has a feature called Optimum Barrel Time that predicts accuracy nodes and I find it not to be exactly accurate but within the feature you can adjust the prediction of velocity to your actual chronograph velocity and it changes the pressure map which I have found to be fairly consistent with my fired brass condition and this should give you comfort in moving up powder charges. I’ve got two long freebore 223’s that shoot 75 and 80 grain bullets at 2980 and 2950 fps. Based on reloading manuals, I would be so overloaded, I’d never had chosen the charge without slowly increasing until pressure and always questioning the brass condition. I can go farther with the velocity at this point but with nearly 3000 fps I’ll probably never get another accuracy node and I’m satisfied. This recommendation is based on actual experience and I’d hate not to share it because it’s a free tool and it’s fun to explore. I love making things go bang but this will increase your confidence and make more shots count.

I never looked at powders as far as burn rate. I look in the manuals and see which powders give the highest fps. This eliminats burn rate charts. It's real info determined with a rifle and not a test cylinder. Never look at % case fill. I tend to pick powders that are not new on the market. I have always bought Kreiger barrels and I never had one that wouldn't shoot under .500" groups with almost any load. Currently use 8208 for all loads. Been varmint hunting for 50 years.

If you look at Jack Neary's video you can see what it really takes to be a top short range competitor. The rest of us are chasing our tails. A full race rifle with a scope cost about $9000. How many guys on this website giving out info can shoot .300-.350" groups consistantly.

 

[Ned Ludd]

Cartridge volume affect powder burn rate, as can its shape. As I understand it, burn rates are typically derived using calorimetry, and that equipment likely doesn't correspond to the size or shape of any cartridges shooters are using. That is why programs such as QuickLoad and Gordon's Reloading Tool provide a single user-adjustable burn rate factor (Ba), such that it can be adjutsed to match the empirical velocity data generated from and infinite number of different cartridge sizes/shapes.

The typical burn rate chart is fairly crude roadmap that will hopefully be helpful in getting a reloader to the correct neighborhood, if not necessarily the correct street address. Nonetheless, powder combustion is a chemical reaction. As such, powder burn rate is affected by pressure. Pressure is affected by cartidge size/shape, the amount of chemical reactant (i.e. powder charge weight), initiation (i.e. primer brisance), and probably a host of other things I haven't mentioned. A powder burn rate chart or pull-down menu in a reloading program cannot possibly address all the variables that might affect the output. So companies give a powder a single position in a burn rate chart relative to other powders, or a single burn rate value in a pull-down menu, and then let the user figure out the rest in their highly specific set of reaction conditions (i.e. their rifle setup).

 

[Webster]

Expansion ratio is the big determiner on on how powders act. -Al

And the time to expand.

 

[JSH]

I was of the thoughts of bore size more than case size.

 

[Rick300]

All the talk above has me wondering about replacing H4198 in a small case like the 20 Vartarg. While RL7 shoots just about as good, I would like to have alternatives in case that goes to unicorn status too.
Do the alternatives discussed in the 30br thread work in a small case also?

 

[RegionRat]

Many of the same powders that work on the light end bullets for BR and 223, also work in 20 VT.
The hard part is guessing which powders will have reliable availability in the future?

Here are some alternative suggestions from the main site 20VT page in case it helps.

And some suggestions from the Hodgdon Page.

 

[Doom]

I too would recommend Quickload or GRT because one of the main requirements for an accurate, repeatable load is case fill. Unfortunately burn rate alone will not give you case fill and both pieces of software will along with adjustments for different cartridges. They also calculate the burnout so it's easy to see which powders burn out in the barrel under consideration.

 

[William Vogel]

Also want to consider that if moving to a heavier and longer bullet, you really need to increase the COL or at least maintain the same internal volume inside the case as with any bullet to allow the maximum efficiency of the round. Sometimes it requires modification of the freebore to keep the bearing surface of the bullet only in the neck past the pressure ring. Doing this will allow for greater velocity, powder capacity and safe pressure.