How to match powder to barrel length?

[Jeffpatton00]

I'm interested in learning the theory around matching a bullet/powder combination to a new barrel. There are some threads with people suggesting different powders to try, but that's not what I'm looking for; instead, I'd like to understand the science or art behind thoughtfully, from data, choosing a powder that best fits the bullet/barrel combination.

I've been shooting a Savage 10 in 6.5 CM, 24" bbl, 1:8 twist, with 140gr, H4350, and I'm moving to a 28" Shilen bbl, 1:8 twist. Conventional wisdom says to start load development again and work up to pressure signs, but it occurs to me that it may be short-sighted to assume that H4350 will be equally effective in a longer barrel. Is it true that there are powders with a better burn rate for a given barrel length? And yes, also matched to a bullet weight?

So, my specific question is, how do experienced reloaders think about different powers on the burn rate chart, as it relates to barrel length? Is it generally true that shorter barrels require a faster powder, and longer barrels a slower one? And if so, then theoretically, moving from a 24" to a 28" bbl might drive a consideration of a slightly slower powder? I know this isn't a large change in barrel length, compared to moving from a 16" to a 28", but still, it's the theory behind all this that I'd like to understand. Again, I'm not looking for powder suggestions, but I'm very interested in how knowledgeable reloaders think about burn rates related to barrel length. thx

 

[coop2564]

tagging in. I know Quick load is a easy solution for a price. It has a % burn rate, based on barrel lenght, powder, etc. So there is probably a math formula out there somewhere?

 

[Seabeeken]

There is a balancing act of case capacity and barrel length. Simply use a powder that will give you the highest speed and fill the case. In your case, H-4350 or 6.5 Staball might be best. You just cant get enough slow burning powder in the case to be of benefit. Just my .02

 

[Seabeeken]

tagging in. I know Quick load is a easy solution for a price. It has a % burn rate, based on barrel lenght, powder, etc. So there is probably a math formula out there somewhere?

There is also an efficiency number in there. Anything 30% or higher is considered outstanding but often means faster burning powders.

 

[lyle2231]

The rule of thumb I learned was a short barrel would shoot best with a faster powder, 18" and shorter.

 

[cohunt]

I'm not a believer of the "faster powder for short barrels" -- I've tried it and in my experience it does nothing.

I believe the burn rate is based off the cartridge and bullet weight.

I have a 16" 308w and a 26" ( also same in 6.8spc and 223/5.56)--- in the 16" I tried a multitude of "faster" powders and could never get faster speeds for the same weight bullet. The faster burn rate powders just show pressure signs earlier in my experience.
Faster powders for lighter bullets- yes, that works for me.

Others may have different results but faster powders for shorter barrels don't work for me.

 

[lyle2231]

I'm not a believer of the "faster powder for short barrels" -- I've tried it and in my experience it does nothing.

I believe the burn rate is based off the cartridge and bullet weight.

I have a 16" 308w and a 26" ( also same in 6.8spc and 223/5.56)--- in the 16" I tried a multitude of "faster" powders and could never get faster speeds for the same weight bullet. The faster burn rate powders just show pressure signs earlier in my experience.
Faster powders for lighter bullets- yes, that works for me.

Others may have different results but faster powders for shorter barrels don't work for me.

Thanks for the comment. It's just what I discovered in the gas gun world.
Just my own observations. Cheers!

 

[jimbires]

another vote for quickload . it will show potential velocity better than anything I've seen .

I read the threads you're talking about . sometimes I'll do a quickload just to see if some of these powders work out to what they say . nope, not even close . or the it's right beside it on a burn chart . sorry , nope again .

 

[Mike D Texas]

Quickload is the easy button for this.

 

[Jeffpatton00]

another vote for quickload . it will show potential velocity better than anything I've seen .

I read the threads you're talking about . sometimes I'll do a quickload just to see if some of these powders work out to what they say . nope, not even close . or the it's right beside it on a burn chart . sorry , nope again .

thx for the tip on Quickload. I'm remembering back in Engineering school, they made us work out the algorithms to make sure we understood the concepts, and only then they'd tell us about the books with charts showing the needed data, without having to work things out mathematically. I'll probably go ahead and pick up Quickload, but it's the theory that I'm hoping to learn more about - Quickload may know all about it, but I'd like to understand the interplay between burn rate, barrel length and bullet mass too. And then, like back in school, I'll let Quickload do the thinking for me.

By the way, one of the justifications for Professors making us understand the theory and mathematics was so that, when we started using the shortcut via the charts, we'd be more able to apply an internal "sanity test" on the results the charts gave. There's a lot of value in having a sense of when results don't make sense when you're designing a bridge or an airplane, and maybe even more when you're dealing with things that can blow up in my face.

 

[yorke-1]

I'm 100% with Cohunt on this one.

Just for fun, go over to Hodgdon's reloading site and look at the 308 Win data under both rifle and pistol. They list data for both 15" and 24" barrels; it's the same data for both lengths. You'll see the same thing when you compare rifle and pistol data from other sources.

 

[Plinker147]

Yeah I don't believe there is much to matching barrel length with burn rates on powders. Usually the powder that gives the best velocity will still be the best powder with short or long barrels. The statement the "slower burning powders"doesn't fully burn in short barrels isnt really true. What really happens is the gases are trapped for less distances In a short barrel than in a long barrel so you just aren't taking advantage of all the energy created by the burned powder.

matching bullet weights to burn rate or a powder is valid.

 

[QuietTexan]

The statement the "slower burning powders"doesn't fully burn in short barrels isn't really true.

Agree. All it takes is loading up some subs with fast pistol powders and dumping the remnants out of a suppressor to see that there's a lot more at play than just the two variables of burn rate and barrel length, because it's easy to prove that even fast powders don't fully burn in short barrels necessarily. There's at least the third variable of chamber pressure, not as a result of any discrete combination of bullet and powder, but as a minimum target in the equation to achieve maximal efficiency. The length of time the chamber is at or above a certain pressure makes an impact on the burn of the rest of the powder.

I'd like to understand the science or art behind thoughtfully, from data, choosing a powder that best fits the bullet/barrel combination.

Pour powder into a bowl, light it on fire, watch it burn for 1.5 seconds. Put the same powder in a case and it burns almost instantly. The pressure produced by the first burning grain in the contained case reduces the time it takes each successive grain to burn, because each successive grain starts burning at a higher temperature and pressure. Each next grain burns faster, and the chain reaction takes off. The byproduct of each successive kernel burning is creating a positive-feedback loop.

Burn rate might be linear at one atmosphere, but if you add in the increase in pressure because the burn is contained, pressure grows exponentially upon itself resulting in the upside of the chamber pressure curve. Eventually, either by the reacting mass of the remaining powder decreasing, the bullet moving down the barrel and increasing the volume of the container the powder is burning in, or more really both of those combined, pressure drops exponentially again.

We use fast powders in 300 BLK subs so that the combustion happens fast enough to get pressure high enough to move the bullet all the way up to a speed, and even then the burn isn't always complete because pressure starts dropping as the bullet moves. If you put a slow powder in the case pressure might not ever get high enough to move the bullet all the way down the barrel - because there wasn't enough exponential growth in pressure to burn all the powder quickly enough to generate enough gas volume to move the bullet out. If the pressure doesn't grow exponentially, the rate kernels burn at never accelerates fast enough to do anything but leak out of the chamber around the case that's not sealed to the chamber wall. Or more commonly it blows up the action. Time matters a lot here, you can make a pipe bomb out of an action with too little powder just as easily as with too much. There's really no such thing as a safe amount of undercharging, to get down to the "leaks around the case" point you're basically at like three kernels of H1000 in a 300 RUM with a primer level of pressure. You get up to a few dozen kernels and yeah, you're going to get gas to the face, a cut up bolt face, and maybe a blown out barrel tenon.

The science here is choosing a powder that at a certain mass, as defined by the size of the case, as expanded by the size of the bore, under timing dictated by the mass of the projectile, achieves maximum peak chamber pressure. QL does this using math.

Even then the variables of throating/seating depth, bore condition, how the bullet and case seal to the bore and chamber, ignition timing resulting from brisance of the primer, flash hose size, case dimensions, and a few dozen other things all make some level of impact that can't be input as variables and are assumed to be constant. Even with all the math, there's no way short of experimentally determining the most efficient load because those variables, minor as they might be, have to be accounted for. That doesn't even get into the exterior factors that impact the result of interior ballistics, like barrel harmonics, and action bedding, and a million other things that control if the most efficient load turns into the most precise and accurate load.

The art is doing all that experimentally with the imperfect data we have, using a sample size less than what is really needed to produce inferential results. Basically guess really, really good. We do this using past experience.

It's a lot easier to start with things that have been shown to work well experimentally than rebuilding the wheel from scratch. This is why I advise people when the time comes to play with burn rates of powders to do so inside on manufacturer's products. The jumps from H380 to H4350 to H4831 should always be incrementally faster if the powders are of similar time frames. There's no direct cross-comparability between manufacturers, and any two given pound of H4350 could be faster or slower than any pound of IMR 4350. Sure RL 33 is going to be slower than Varget, but what case/bullet combo are you really directly comparing two such drastically different powders in?

You can test powder from multiple manufacturers to try to really tune down around a burn rate, but ultimately that tuning is unique to the physical cans of powder that you have at that time, and might not be replicated lot-to-lot.

 

[DUSTY NOGGIN]

cant really add to that