Fill Percentage vs. Burn Percentage

[rustyinbend]

Which is more important to get near 100%? Fill Percentage or Burn Percentage? I'm building my first ladder tests for the 338-LM barrel on my Barrett MRAD. One combination of powder and projectile gets me one way or the other (using QuickLOAD) ... based upon my supplies on-hand. If I have one at 98% and the other at 90% ... which one do I want to be higher? Any advice from the Reloading-Guru's out there? Is there a Fill Percentage that I should never go below?

 

[alamo5000]

It would depend on what you want to accomplish but I don't think one is more important than the other per se. If I had to err on the side of one or the other, I personally would go with burn percentage.

 

[Choid]

Which is more important to get near 100%? Fill Percentage or Burn Percentage? I'm building my first ladder tests for the 338-LM barrel on my Barrett MRAD. One combination of powder and projectile gets me one way or the other (using QuickLOAD) ... based upon my supplies on-hand. If I have one at 98% and the other at 90% ... which one do I want to be higher? Any advice from the Reloading-Guru's out there? Is there a Fill Percentage that I should never go below?

85% is what you really want to stay above for fill percentage to make sure you stay far away from risking detonation.

 

[Steel head]

My current 338 load is 100% burn (at 26.1”) snd 98% fill.

No scientific evidence but it always seems like a pretty full case with every a tiny bit of powder crunch always gives me a pretty decent extreme spread.

 

[Highbrass]

I have noticed a trend in several calibers I load for that I see in Quickload. (This may be seen as Vudoo magic) Do the H20 case measurement so the volume is a known. Choose your bullet. Then do a powder search that gives 95+ % fill and close to 100% burn rate. More often than not this combination will give an accurate load.
I have had better than average luck with this combination with the variable being seating depth. My start point is .01 off the lands.
My uneducated guess is that @ 95% fill the projectile will be resting on the powder column. And this acts as a shock absorber.
I also have a 7-08 load that got better the faster I pushed it...
I have noticed that the 95% fill is a very good starting point and is usually predictable. YMMV...

 

[Choid]

In general high fill is intuitively the way to go because there are fewer ways for density to be inconsistent as fill percentage gets higher. I think the 300 PRC/RL26 combo has put some fuzz in the machine as far as approaching this, at least for me, because it is such a potent and accepted combo at a relatively low fill percentage. I can't see any upside to a low percentage of burnt propellant.

 

[brianf]

My current 338 load is 100% burn (at 26.1”) snd 98% fill.

No scientific evidence but it always seems like a pretty full case with every a tiny bit of powder crunch always gives me a pretty decent extreme spread.

not sure howd you know its 100% burn unless you shoot into a "container" and capture all the material that comes out of the barrel

but im with you on case fill is prob more important than total burn

unless your trying to be super efficient with the amount of powder used for a given speed etc

 

[Steel head]

not sure howd you know its 100% burn unless you shoot into a "container" and capture all the material that comes out of the barrel

but im with you on case fill is prob more important than total burn

unless your trying to be super efficient with the amount of powder used for a given speed etc

Just going by what my internal ballistics software shows

 

[Cascade Hemi]

In load work up I try to stay over 90% fill. I have never considered burn percentage when choosing a powder for a bolt gun.

 

[rustyinbend]

Taking out ladder test loads today for my 338-LM as follows:

250gr Berger - Start/End Fill: 91%/94% - Start/End Burn: 95%/98%
300gr Berger - Start/End Fill: 88%/91% - Start/End Burn: 97%/98%
285gr Hornady - Start/End Fill: 88%/91% - Start/End Burn: 97%/98%

Did the calculations on QuickLoad to make the last shot in each 10-round ladder to be the first "Near Max" warning ... with charge increasing by 0.3gr consecutively. Using Lapua never-fired brass, Accurate MagPro powder, and Federal Premium Magnum primers.

Let's see if I can catch a Velocity Node in one of these, and also looking for pressure signs. This is my first attempt at Load Development on my 338-LM Barrett MRAD barrel. Let's see where it ends up. Kind of hoping the 300-Bergers run well ... since I have a boatload of them.

If good results and no pressure signs, I'll take the next ladder up ... which will only help increase both fill and burn.

I'll share my results when I get home later today. I'm really interested to see if the lower fill percentages (88-ish) still shoot accurately.

 

[Huskydriver]

What powder?

 

[rustyinbend]

What powder?

Accurate MagPro

 

[Huskydriver]

Accurate MagPro

Yuck

 

[rustyinbend]

Yuck

I've actually gotten solid results with it in my 300-WM ... settled on a load using 200.20x-Bergers that gives me sub-6.0 SD's and tight groups. That said, this is my first attempt in 338-LM. Hoping it continues to work well, since I have over 30 lbs of it on the shelf. In these crazy times, you've gotta use what you have on the shelf. Would love to find some Reloder-33 or H1000 ... but I'd also like to ride a Unicorn and film Big Foot going through my trash barrel.

 

[Dogtown]

Generally, I agree that we should be looking at a high fill % / burn % ratio when choosing a powder. In a lot of ways, when those both approach 100%, then you've got an extremely efficient load. And from my own experience, if I'm getting powder burn %'s below 95% that's usually a sign that the powder I'm using is just too slow for those components or I'm just using a short barrel. For example, I've got a .338LM load that burns about 99% in a 28" barrel but 95% in an 18". Nothing's wrong with that, but just something to be aware of.

 

[Long Range 338]

Most reloading manuals make mention of seeing the best consistency around 95% case fill, especially on larger cases such as magnums. I'd wager most cartridges can't burn that much powder in the typical barrel lengths.