Sorry it has taken me so long to answer. For some reason, I don't get notification of responses to threads on this forum.
DuPont IMR 4350, and others, was originally made in the US. The physical tube size of what they extruded was larger than the Canadian variety DuPont also offered in the 1980s and later.
A year or so ago, I weighed a full case of powder in R-P 6mm, using different powders from different vintages.
H4831SC 53.5 gr
1974 DuPont IMR 4831 48.4 gr
2003 IMR IMR 4831 52.3 gr
1973 DuPont IMR 3031 47.2 gr
2003 IMR IMR 3031 49.7 gr.
The reason I noticed this was during a loading session of duplicating 7mm Mag loads we had been using since about 1965. My dad pretty well insisted on a case full of powder and what I loaded with IMR IMR 4350 didn't come up in the shoulder very far. That was very uncharacteristic, so I checked further. The newer IMR 4350 shoots within 10 fps of the old powder (in limited testing), but uses less case capacity for the same weight of powder.
A cap full of powder from each of the old metal cans clearly shows a difference in tube size.
