Slow Burning Power VS Fast Burning Power?

[SheepDog]

Burn rate charts are put together using a specific weight of powder in a closed container of a given size under laboratory conditions. It's referred to as a closed bomb test. The container holds all the pressure generated by the burning propellant and records the pressure rise time. The problem with using this data in a gun is that the volume of the chamber in a gun increases when the powder is ignited. This causes the burn rate to change. The burn rate will also change with different case capacities and bullet weights, not to mention temperatures, both ambient and chamber temps.
We rely on published load data because we have no way to measure the actual pressure rise times in our gun. The good folks that make the manuals have that equipment and share their findings with us so that we can put together relatively safe ammo.

 

[XTR]

All of the above is why I now use a pressure trace when I'm working up competition loads. It may not be calibrated, but it certainly lets me compare apples to apples when I am evaluating powders.

 

[ebb]

I was at the range a few years ago and a shooter had a Remington semi auto rifle in 308. I had seen him shoot for years and he had developed a few loads for his rifle and it shot better than I would have thought. As I walked up to him I noticed that the sides of the receiver were bulged and parts pins and springs were falling out the bulges. Nothing on the rifle could be saved not scope not rings not stock. He had made a mistake and used 40 some grains of FAST pistol powder instead of reloader 15. Be careful how fast you go!

 

[Laurie]

Burn rate charts are put together using a specific weight of powder in a closed container of a given size under laboratory conditions. It's referred to as a closed bomb test. The container holds all the pressure generated by the burning propellant and records the pressure rise time. The problem with using this data in a gun is that the volume of the chamber in a gun increases when the powder is ignited. This causes the burn rate to change. The burn rate will also change with different case capacities and bullet weights, not to mention temperatures, both ambient and chamber temps.

An interesting bit of test kit and procedure - it goes back a long way too. Whilst it is obviously valuable to the manufacturer to be able to rate its products in ascending relative rates, the closed bomb test unfortunately fails to provide any sort of absolute standard that allows reliable assessment across the market by users. No doubt this comes back to the issue of other 'real life' factors affecting the burning rate as Sheepdog says, but there also appears to be a degree of interpretation or modification put onto some products by their producers. Otherwise, I can't see as to how charts from different sources provide such different results for some products when in theory they have all been tested in a standard way in a standard apparatus.

Some ball powders too simply don't seem to fit well into standard charts and are also apparently more affected by the internal ballistic characteristics of individual applications. The old (goes back to WW2) Hodgdon BL-C(2) has over the years I've been interested in handloading been shown as relatively fast burning in relation to extruded standards such as the 4895s, or alternatively on a par with them, or sometimes slower burning. Hodgdon Powder Co. has IMHO been very consistent and honest in its descriptions of its products over a long period, but over the years this particular grade seems to move around even in the company's own list. Right now it and the younger but improved CFE223 variant are shown together as numbers 104 and 105 in the company's list as slower than H4895. IMR-4895, Re15, VarGet, Viht N140. For many years it was shown as faster burning than all of these others. Viht's very useful chart still shows it as equivalent to Viht N130 and faster burning than all those I've just mentioned, in other words the more 'traditional' position.

http://www.lapua.com/upload/reloading/reloadingburningratechart2011.pdf

 

[XTR]

Laurie, some day I may have enough information from my pressure trace testing to put together a useful document; however, to date I've only been using the pressure trace for a short while. I will say that what I am seeing has caused me to reconsider my thoughts on a couple of things with regard to what happens with pressures.

 

[fguffey]

I will say that what I am seeing has caused me to reconsider my thoughts on a couple of things with regard to what happens with pressures.

Meaning? There are things. events and sequences that have not been discovered? I am not waiting for someone to claim they reinvented the events between pulling the trigger and the bullet leaving the barrel.

F. Guffey

 

[.22gongzilla]

...how do you determine between the various powders as to which ones are considered slow burning and...

...I've always used the Hodgdon burn rate chart, but there are others. My Lee reloading manual also has burn rate listings alongside the load data.

 

[SheepDog]

To determine a slow vs a fast powder for my bullet and cartridge I use the manual. Slower burning powders will give higher velocities with more powder right up to where they run out of room in the case. The three highest velocities listed are likely to be the three slowest powders for a given case and bullet.

 

[K22]

Some manuals give you the most accurate powder they tested for each caliber / bullet weight, i.e. Lyman, Nosler, Sierra to name a few. This a good place to start but isn't absolute.

Some cartridges are more forgiving than others and the weight of the bullet you select also can be a factor. In my experience, the faster powders work well with the lighter bullets whereas the slower ones work better with heaiver bullets for a given caliber. For example in the 243 Win, IMR 4064 or Varget with 80 grain bullets and under; IMR 4350 for 90 grain and heavier. But again this is not absolute.

Burn charts mentioned by other will give you an indication of relative burn rates.