Powder grains are like snowflakes, there are no two alike. As there are no two alike then no two grains of powder burn the same.
Since no two burn the same unless something is done gun IV would be very different with every round fired.
To correct this problem the powder is mixed until the entire lot of powder or any portion of the lot will burn at the same rate when fired in the same gun with the same weight projectile.
The mixing of the powder results in a uniform mix of all of the different grains of powder throughout the lot of powder. Thus each gram of powder, each pound of powder and each charge of powder from the same lot contain an exact mix of all of the different grains of powder and each gram, pound and charge of powder from the same lot burns exactly alike. When fired in the same gun with the same weight of projectile the IV produced will be exactly the same from round to round.
At the factory making the powder it is mixed until they feel that it is well mixed and will preform as required by the USN. The powder from one manufacturing run is identified by number and stored. A sample of this powder is sent to Dahlgren for testing.
At Dahlgren if the powder meets with the USN specifications a lot number is assigned by Dahlgren and the powder is then labeled with this number.
If the powder fails to meet USN specifications then the factory is notified and takes the necessary steps of re-mixing and blending of the powder. Then a new sample is sent to Dahlgren for testing.
Only after the powder meets the USN specifications is the powder issued a lot number.
Each lot of powder is manufactured for one specific gun. After the powder meets the USN specifications it is stored until BuOrd orders the powder shipped and assembled into bag or case ammunition as required.
Powder is expensive to make and none is ever thrown away. After the bag or case ammunition is assembled any powder left over is stored and eventually will be shipped back to the factory when more powder is ordered by BuOrd.
This powder can be re-mixed with new powder of the next lot manufactured so that no waste occurs.
It is also possible to mix powder from several existing lots and obtain a powder that meets the USN specifications.
Of course all lots of powder must be tested at Dahlgren and approved for the specified gun the powder is being made for.
A good case of this mixing of different batches of powder is the powder used in the 1980 tests that everyone seems to think the results are so accurate.
The powder came from lots of powder left over from WWII and the Korean war and were remixed during the 1960's and used in Vietnam. The powder was not reground before mixing.
These lots of powder were treated like all USN powder and were properly stored for years before use.
The USN stores powder in both ammunition depots as well as barges designed for the task. The barges are built as the powder magazines on ships and use the same aeration method as aboard ship and the powder is tested the same as aboard ship to insure its quality over time.
About 1935 the USN changed its requirements for powder to the design velocity of the gun and projectile.
In the design of a 16"/50 gun the powder charge in the design was for 650 pounds of powder that would produce an IV of 2500 FPS with a standard weight 2700 pound projectile in a new gun.
Assume the gun design requires 2500fps powder, then all lots of powder for this gun must fall into 2500fps plus or minus 10fps or a range of 2490 to 2510fps.
This means that when powder from the same lot is fired in the same gun with a projectile of the same weight the projectile or gun IV will be the same for all rounds fired.
Let us say then that a particular batch of powder and a full charge of this powder weighing 650 pounds is fired in a new gun with a 2700 pound projectile with each firing the gun or projectile IV will be 2495fps for each and every round so fired.
If however you change to another batch of powder the new batch of powder might produce a gun and projectile IV of say 2505fps in the same gun. But every round fired with this new powder in the same gun with the same weight projectile will yield 2505fps velocity.
The allowed plus or minus 10fps variation does not mean that all rounds fired will vary within plus or minus 10fps of the gun design. This would make for some very wild patterns and simply does not occur.
To correct the variation in powder IV so that each powder charge from different batches of powder will produce 2500FPS IV in a new gun when firing a standard 2700 pound projectile Dahlgren how many pounds of powder that will be required for each batch of powder so that a full powder charge will produce 2500FPS IV in a new gun with a 2700 pound projectile.
Thus each lot of powder will have a different weight for a full charge and a full powder charge usually does not weigh 650 pounds.
THE DATA YOU SEE PRINTED ON THE POWDER BAG TELLS YOU WHICH GUN THE POWDER CHARGE IS FOR.
THE LOT OR INDEX NUMBER THAT THIS POWDER CAME FROM.
HOW MANY BAGS ARE REQUIRED FOR A FULL CHARGE.
HOW MUCH THE FULL CHARGE WEIGHS.
WHAT THE IV WILL BE IN A STANDARD GUN WITH A STANDARD PROJECTILE.
EVERY LOT OF POWDER WILL HAVE A DIFFERENT WEIGHT OF POWDER FOR A FULL CHARGE.
IN OTHER WORDS IN A LOT OF POWDER A FULL CHARGE WILL WEIGH 665 POUNDS FOR EVERY FULL POWDER CHARGE IN THAT LOT. BUT THIS LOT OF POWDER WILL PRODUCE THE IV WRITTEN ON THE POWDER BAG.
IN THE NEXT LOT OF POWDER A FULL CHARGE MAY WEIGH 670 POUNDS FOR EVERY FULL POWDER CHARGE IN THAT LOT. THIS LOT AS WELL WILL PRODUCE THE IV WRITTEN ON THE BAG.
NO MATTER WHAT THE WEIGHT OF THE POWDER CHARGE OR WHICH LOT THE POWDER CHARGE COMES FROM EACH LOT WILL ALWAYS PRODUCE THE IV WRITTEN ON THE POWDER BAG.
THIS MEANS THAT WHEN YOU CHANGE LOTS OF POWDER A STANDARD GUN WITH A STANDARD PROJECTILE WILL PRODUCE THE SAME IV.
IN OTHER WORDS ALL POWDER MADE FOR A GUN NO MATTER WHICH LOT OF POWDER IT COMES FROM WILL PRODUCE THE SAME IV.
If you fire 2 standard guns with standard projectiles at the same time at the same time but each gun with a different lot of powder the IV will be the same for each gun.
If you mix powder bags from different lots the IV will still be the same as when the entire charge is from the same lot.
Prior to this the USN allowance for variation was plus or minus 25fps.
The tightening of the standard to plus or minus 10fps reduced the variation in weights of a full powder charge between different lots of powder.