Interesting article on powder
What's in a number? IMR vs. Hodgdon
Guns Magazine, Dec, 2005 by Charles E. Petty
When you go to buy a can of powder you will often see powders from different manufacturers with the same number or one very similar. One of the easiest to see is a comparison between IMR and Hodgdon. You can find IMR 4895 and Hodgdon 4895 (or 4350 or 4831) and it would not be unreasonable to think they were interchangeable. They are not.
We are told to use loading data only for the specific powder we have and I've taken that for granted, but I've always wondered what the real differences might be. The logical conclusion is to load identical ammo changing only the powder make. Some of the most useful powders in our inventory are 4895, 4831 and 4350. Both Hodgdon and IMR use exactly the same nomenclature and the powders are similar in application so I chose three calibers for which I had accurate rifles and lots of loading experience. Those were a Cooper Montana Varminter .22-250, a Remington 40X in .260 Remington and a Savage 116 in .300 WSM.
Based on my history with those rifles 1 had developed distinct powder preferences for each. The .22-250 did well with 4895, the .260 liked 4831 and the .300 WSM did best with 4350. So using data from several sources 1 chose charge weights from starting to near maximum for each with bullets of typical weights for the cartridge. Most data had a spread of two grains from high to low so a total of four charges, in half-grain increments were loaded for each one.
Since about 1990 Hodgdon has had a very fruitful relationship with ADI and some of their best powders such as Clays, Varget and Trail Boss come from there. The first thing you notice about them is they are not black. Most powders we are used to are liberally coated with graphite to help control the burning rate, but according to Ron Reiber, Hodgdon's ballistician, ADI's process control is so good they only use a small amount of graphite, so we see a powder with a color much closer to that of plain nitrocelhdose.
It is entirely reasonable to wonder why--since the names are the same--the powders aren't. It is all because of the cussed randomness of organic chemistry and the whole firearm system. Even with the best possible process control, organic reactions vary with things like temperature, humidity the phases of the moon and whether they had a bad night.
Extruded powders are usually manufactured in batches of around 5,000 to 15,000 pounds at a time. When done they are tested against a reference lot. If a particular batch does not fall within the specifications for a specific powder name or number the manufacturers are able to blend with lots that may be a little slower or faster to get a lot of powder that falls within specifications. When the powder gets to Hodgdon they check it again against the same reference lot. Both reference and new powders are shot with the same components and at the same conditions of temperature and humidity. A number of 10-shot strings are fired in SAAMI standard test equipment and both pressure and velocity are measured. If the new powder is within +/- 3 percent of the reference lot it is accepted and packaged.
My guess is some of you are thinking a 3 percent tolerance is pretty big. Not so. In fact it is a very tight standard when you remember the variability of our system. It really isn't too rare to see a 10-percent variation in either pressure or velocity with a not-so-hot load. In the laboratory, variables can be controlled far better than we could ever hope to, whereas we are at the mercy of the weather and all of the different variables of our own production guns.
Of course many reloaders look at the data in somebody's manual and assume that they will get exactly the same thing down to the nearest tenth of a foot per second when the fact is we are lucky to come within plus or minus 25. Velocities in manuals are just a guide and the only way we can ever know what our loads are doing is to chronograph them ourselves. If you really want to get frustrated, shoot the same load, in the same gun, during each of the four seasons or in climatological extremes of temperature and humidity.
One of the other things I did in the course of this test was look at accuracy. Since each data point only represents one five-shot group, we can't draw conclusions about best anything, but there is an interesting observation. Comparing group sizes for the same charge weight between IMR and Hodgdon, sometimes the IMR group would be a little smaller and sometimes Hodgdon would win. But when we look at the big picture there really isn't anything on which we can hang our hats. Just as we saw with the velocity measurements, sometimes Hodgdon was faster and sometimes it wasn't. And sometimes--just to jerk our chain a bit--there wasn't any real difference.