JJ: the problem is in multiple parts so when you see someone say that burn rate charts are for reference only and not concise enough to form any opinions on, it's a true statement. Once upon a time in the gun rag west, I've seen recommendations for going to the five powders ahead of the one you're using, to find a start charge. In some cases it works, but it's not a very good idea. Powder manufacturers don't produce them like they used to and when they do, they tend to place their own powders as much for marketing reasons as much as the actual results from closed bomb testing. They will be very close even if their powder is off by a place or two and they are still useful, but its best to go to the specific cartridge data and see what charges are for that particular load. Loads with higher charges most often indicate a slower burn rate for rifle powders, but not always for handgun. Clear as mud ain't it? Some handgun powders have a slower burn rate than what's indicated by chargeweight. Typically they are capable of producing higher velocity with a given bullet, even though the powder charge is lower. It's not a lot of them, but Vihta Vouri makes a few examples like 3N37 and N350. Mostly it's chemistry and there are differences and they are explained in most load manuals. Some like Accurate's vary by little more than nitroglycerin content and with others it gets into single and double based powder issues.
Some of the independent charts are very good, but I have yet to see one where you don't find an anomaly or two. By and large, the burn rate charts the powder companies produce are as good as any of them and if you're going to use them, use several and cross reference. If the first one doesn't agree with four others you're looking at, there's an 80% chance that the first might have misplaced a powder in the rankings. Vihta Vouri used to produce a pretty decent chart and Ramshot has one of the more recent ones and several are listed in load manuals. Unfortunately, some of the biggest discrepencies I have seen have been in load manuals, but they usually don't do their own testing. Neither do the guys that create their own charts in many cases. They do theirs by experience and cross referencing. I've got one of the most inclusive ones available and if you'd like to PM me with an e-mail address I'd be glad to send it to you in pdf format, but I can tell you that it has discrepancies also. lot's of help, right?
Ideally, powders shouldn't vary, but quality control is not 100% with anything. Sometimes powders do vary and if it is a significant variance the powder will be recalled by the lot number that's printed on the canister. Some powders get reformulated and it is pretty rare. The most recent case is Unique to achieve a cleaner burning product. Alliant's goal was to reformulate without a need for any change of existing data. They did pretty well considering that Unique was around 100 years old when it got reformulated. Is it exact? I can't say for sure because I haven't used it much in a good number of years. My guess is that it varies no more than .1 or .2 grains from previous data in extreme cases such as a load at its maximum average pressure, MAP. And you have it right. In some cases they're just too close to call and powders separated by 1 place on the chart isn't much of a discrepancy anyway.
