The issue at hand is the difference in density (bulk density in this case) between lots of the same powder. I'll define a couple of terms to start with.
Density = Mass per unit volume - is the formal definition. Let's not get hung up on "mass" versus "weight". Technically, "mass" is independent of gravity, where "weight" is dependent on gravity. For example: A dumbell that weighs 10 lbs on earth would weigh about 1.7 lbs on the moon. That same dumbell would have the same mass on the earth as on the moon - ~4.54 kilograms (or in "British" units, 0.311 slugs). This independence from the effect of gravity is why scientists and engineers prefer "mass" to "weight" because it relieves them of the task of calculating "weight" as a function of variations in gravity. HOWEVER, we all do our shooting on earth, so weight is just fine for our purposes.
"Bulk" Density - This is the density of a material that is commonly used in "bulk" but occurs in tiny pieces - like gunpowder. We use thousands of "pieces" (the granules) of gunpowder in a single charge. One granule of the powder has a specific density, (as defined above), but one grain, (weight grain), of the powder has a different "bulk" density due to all of the "air" between all the individual granules of that one grain (weight). Put another way, if we had a grain, (weight "grain" as opposed to a 'granule' or one piece), of solid gunpowder, we could measure it's density because that one grain, (weight), would occupy a specific volume. But since one grain, (weight), of powder will contain many granules, the one grain, (weight), of powder will occupy a larger volume. Therefore, its "bulk" density will be less than its "solid" density.
So.. what does this mean to us handloaders?
Since the easiest way to determine the "proper" charge for a specific cartridge is to weigh it, that's what we do. Hence charges are reported usually in "grains" of gunpowder. (The unit "grains" - one seven thousandths of a pound - is an entirely separate issue. ) However, we weigh them, only because this is easy for the "regular Joe" to accomplish because it's easy and cheap to buy scales. It's NOT easy OR cheap to procure the equipment to measure mass per unit volume - AKA density.
HOWEVER, when we take the "easy" path, we give up some 'control' because we have to make some assumptions. The biggest of those assumptions in this matter is that the "bulk" density of the powder in question is always the same. That assumption, while useful, and for the most part necessary, adds variability to our results. The key to understanding why, lies in the fact that gunpowder has a characteristic described as "energy density". (There's that "density" term again.)
Energy Density is the amount of chemical energy (usually reported in units of Joules or Watts) per unit weight. So for example, Vihtavouri Oy, (being the ONLY manufacturer that reports their powder's energy contents), says that N-150 has an energy content of 3750 Joules/gram. They report that the bulk density of N-150 is 0.910 grams per cubic centimeter. Therefore, one can calculate what the energy content of a specific load is. This is important relative to case capacity (volume), and projectile mass (weight). The significance of this is that all those "weights" eveyone uses to determine "appropriate" charges are based on the assumption of a uniform, and constant, "bulk" density figure for a specific powder. That assumption is practical, but weak.
Manufacturing processes, no matter how precise, cannot produce EXACTLY the same energy density from lot to lot of powder. They just can't. Furthermore, the age of the powder, (time since manufacture), also effects density, which in turn effects "bulk" density. What this variability means, is that one man's 50.0 grains of N-150 might very well contain more chemical energy than the next man's 50.0 grains of N-150.
So exactly how big a deal is this really?
Is it just an exercise in meaningless precision?
A tempest in a teapot?
I'll provide some numbers, and let you decide for yourselves.
Where this comes into very clear view, is in the use of QuickLoad reloading programs. Each powder is described in very specific detail regarding its burn characteristics, and its "solid" and "bulk" densities. This is so, because QL uses multivariate equations that require specific powder information in order to precisely and accurately estimate muzzle velocites, chamber pressures, and many other internal ballistic outputs. (This is why, by the way, I always ask people to measure the volume of their cases when they want me to work up preceise QL loads for them.) So what actual, practical effect might lot-to-lot differences in a powder's "bulk" density make?
I was having a tough time trying to find a precise load for a new wildcat I made. QL had I3031 as the powder that provided the most velocity for the least pressure. However, I was "all over the place" on the target when trying to work up loads. Finally, I got a load that was "good eough" to take on a long-distance deer and elk hunt. The first shot out of the rifle in Colorado blew a primer. The primer literally fell out of the case. It wasn't a fluke. When I test-fired the rifle later, it did the same thing. For the life of me I couldn't figure out what was wrong. I KNEW the loads weren't "hot" because I don't load "hot". To make a long story shorter, that batch of powder was probaly 20 years old. While it was "fine" chemically, I am quite certan that it was considerably less dense than "new" powder. What that meant was that a 50 grain charge of "old" powder had WAY more energy than a 50 grain charge of "new" powder because the density of the old powder was WAY less than the density of new powder which in turn meant that the energy density was way MORE in the old powder.
As a result of that series of events, I made a device for measuring the bulk density of gunpowder. It's simple really, just a "tube" of known volume with a little base on it so it stands up. I fill it with the powder in question; pour the powder onto my scale and weigh it; then divide the weight by the volume of the tube.
So here's a specific example using Vihtavouri's N-150. The bulk density according to QuickLoad is 0.868 grams/cc. According to Vihtavouri's reloading manual, the energy content of N-150 is 3750 j/gram. So, 50 grains of N-150 would have a chemical energy content of 12,152 Joules, (50 grains/15.43 grains/gram * 3750 j/gram). When I measure the bulk density of the N-150 I have on hand, I get a figure of 0.812 grams/cc. A difference of about 7%. So when I weigh out 50 grains of my N-150 - because it is lighter than new N-150 - I actually put about 7% more energy in that 50 grain charge because it takes more of my "light density" N-150 to equal the weight of "normal" N-150. What that means is my 50-grain charge is the equivalent, energy-content-wise, of a 53.7 grain charge of "normal" or QuickLoad N-150.
Let me assure you the "lightness" of the bulk density of my N-150 isn't because it's "old". It's only about a year old. It IS because it is a different batch.
It is not my intent to scare anyone. This difference I have measured is the greatest I have measured so far in some 20-odd powders. Most of my other powders are within 3% or so.
On a practical note, let me offer a simple way to check your powder batches. Take a cartridge case - the bigger the better, but even a .45 ACP would work. Fill it with the powder in question. Fill it right to the top to the point that you have to use a straight edge to level off the excess. Weigh that caseful, and write down the weight on the cannister. Then, when you get a new batch of that same powder, do the same thing and compare the weights. If they are the same, then your "pet loads" will continue to work as they always have. However, if there is a difference, then you'll need to adjust your charges to reflect that difference. Using the N-150 example above, I would adjust my "normal" 50-grain charge down to 47.8grains. That would give me the same chemical energy content as my "new powder" 50-grain charges did.
When the variability between the case volumes, (even those with the same headstamp and from the same batch), and the variability of energy density between lots of powder is taken into account, the fact that we can ever shoot tiny groups is a bit surprizing. It points to the "robustness" of the "system". :biggthumpup:
Actually, I did want to explain why the bulk density of a given lot of powder will change over time.
It is NOT a chemical "degradation" of the nitrocellulose.
Nitrocellulose, (to put a "general" name to the complex compound today's powders are), is a fairly stable compound when compared to MANY other commonly used compounds. Based on personal experience and opinion, I'd say the shelf-life of nitrocellulose "gunpowder" - when properly stored - is at least one human life-time, and maybe considerably more. If the powder gets even damp however, it degrades immediately. "Damp" can mean just high humidity.
If that seems to fly in the face of my own observation regarding the cannisters of I3031 I mentioned above, let me explain.
When powder is manufactured, a few chemicals used as "carriers" and as "driers" are added. (For example, one of the driers is ether.) During standard manufacturing processes, these carriers and driers are evaporated off. In the finished product, they are technically "gone". By necessity, that "technically gone" has a practicality to it. Manufacturers cannot wait "forever" for "all" of the carriers and driers to evaporate off, so an "acceptable" level is allowed in the final product.
These volatile chemicals, are readily apparent to your nose when you open a new cannister of powder. However, if you use that powder sparingly, AND if the cannister in which it is contained isn't pretty seriously air-tight, these volatiles will continue to evaporate, and every time you open the cannister, that amount of the volatiles that have evaporated into the air space above the powder escapes. Not only does what is in the air above the powder escape, that escapement "makes room for", (in chemical terms, "lowers the partial pressure") of those compounds in the cannister, making "room" for more of the compound to evaporate out of the granules of powder. This makes the powder more and more energy dense. Every time you open the cannister, you change the bulk density (and therefore the energy density), just a tiny bit. The longer you keep the powder, and the more you open the cannister, the more it will change.
This is why I have mixed emotions regarding 8-lb "kegs" of powder. On one hand, an 8-lb keg ensures that all the powder came from one batch. However, if that powder isn't loaded into cartridges, (which is the absolute BEST place to "store" powder), fairly quickly, opening and closing the container, will over time increase the energy density of that powder.. So for me, 8-lb 'kegs' are great for pistol cartridges in which it might be possible to use 8 lbs in less than a year. But for rifle cartridges, for which even a single pound might last several years, an 8-lb keg would definitely need to have its bulk density monitored. "Monitoring" just means performing the "case full of powder" exercise about once a year.
One might suggest that a reduction in charge would also represent a reduction in volume and thereby change chamber pressure because of an increase in the unoccupied volume of the combustion chamber. Which in turn would represent a "change" in group size, muzzle velocity or point of impact. When the volume of the charge is reduced, the volume of the air in the combustion chamber is increased. Because modern gunpowder's burning characteristics are a function of the pressure in which the powder is burning, increasing the volume of the air in the combustion chamber should slow the burn rate thereby reducing the max pressure, and thereby reducing the muzzle velocity. HOWEVER...
We're back to the TRUE issue which is the relationship between weight and volume. As you may have noted, the volume of the air in the combustion chamber is increased, but... the energy density has not been reduced. Remember, the real issue was the fact that the weight of the charge had to change, not the volume. Of course when we reduce the weight, we reduce the volume, but not by the SAME amount.
Reducing the weight of the 50-grain charge by 3.5 grains (about 7%), does NOT increase the volume of the air in the combustion chamber buy that same 7%. Put mathematically, the effect on pressure is not linearly corellated to a reduction in charge weight if the energy density remains the same.
The volume of air in the combustion chamber went up by, I'd guess, much less than 1%. As a result, there is almost no detectable - at least by me - reduction in muzzle velocity because the energy density of the charge remains the same. So in effect, we have the same amount of chemical energy available, but we're going to burn it in a combustoin chamber that has only very slightly more "air". Therefore, we're not likely to be able to detect (measure) a difference in MV.
Also keep in mind, that 7% is the largest discrepancy in bulk density I have so far measured. Most are half that or less, which further reduces the amount the combustion chamber is increased.
Here's an easy process for determining the actual bulk density of your powders:
Leave the primer in a fired case.
Weigh empty case.
Fill with powder of choice.
Subtract empty case weight from full case weight.
That's that case's capacity with that powder. Do the same thing in a year, (or when you buy a new cannister), and see if the caseful of powder weighs the same using the SAME case. If not, you might want to adjust your charges accordingly. Or...
Instead of trying to keep track of that case for a year...
Weigh an empty, fired, case (with spent primer in).
Fill with powder of choice.
If interested in other powders, repeat the "fill and weigh" sequence with each one, recording the filled weight for each powder.
Now, fill with water and weigh again.
Subtract the weight of the empty case from the weight of the "wet" case and each of the powder-filled.
The difference between the case's weight dry and empty, and it's weight full of water is the case capacity in cubic centimeters. If you now divide the weights of the powders you weighed, by the weight of the water, (the case capacity in CCs), you will have the bulk density for each powder. With that information, you could throw that specific case away if you wanted, perform the same sequences of "fill and weigh" with a different case, and the bulk density figure would be the same.
Let me give an example with "real" numbers.
The weight of an empty case is 150 grains.
The weight of that case full of I4064 is 206.17 grains.
The weight of that case full of water is 213.61 grains or 13.498 grams.
Therefore, the weight of the I4064 is 56.17 grains or 3.640 grams.
The weight of the water is 63.61 grains or 4.155 grams.
Now, dividing the weight of the powder, by the weight of the water, gives 63.61 grains / 58.28 grains = 0.883 (the units cancel out). Dividing using the metric units gives 3.640 grams / 4.155 grams = 0.883. Again the units cancel.. HOWEVER, there is a "truth" we have yet to employ... 1 gram of water weight equals 1 cubic centimeterof volume, therefore, we can divide 3.640 grams of I4064 by 4.155 CCs of water, and get a bulk density of 0.883 grams/cubic centimeter or 0.833 g/cc as the bulk density of I4064.
Armed with that figure, you can use ANY container to determine if the bulk density of your powder has changed over time, or even if the bulk density of the new batch you got is different from the old batch you just finished.
It's been a long one for sure. I tried to make it as short as I could and still be clear. I hope at least one or two reloaders find the information useful.
PS - There is an important point made below by caorach: Namely, that if you are dealing only with one cartridge and one powder, weighing your charges mitigates issues of changes in bulk density. The energy density of 50 grains of "old" powder will be the same as the energy density of 50 grains of "new" powder because you will not be considering volume in any fashion. See his comments two posts below.
HOWEVER, if you have powder that has "been around" for a while, and you are working up new loads with some program like QuickLoad in which the volume if the case and the bulk density of the powder play important roles in calculating chamber pressure, you could find yourself with "hot" loads if your powder's bulk density isn't very close to the bulk density of that QL is using for that powder. Also, according to the manufacturers, energy density varies from batch to batch within a specific powder. While only an indirect measurement, a change in the bulk density of "new" powder might indicate a change in the energy density of that batch. The manufacturers say that the differences may be as much as 15% and so when you buy a new batch of powder you should start your load workup all over. I think that is bullwash generated either by their lawyers, or their sales staff. A 15% difference in energy density would be HUGE!
Mostly, this is a non-issue for the fellow that reloads primarily to 'save money' and shoots 10 to 20 rounds a year. For those that like to 'fiddle around' a bit more, and find that reloading costs them more than buying ammo over-the-counter, maybe this will get your 'wheels turning'.