have been shooting a 375 h&h with 260 nosler partitions at 2775 fps....loaded some 260 bal.tip
from nosler with same caseings /load /primer/
powder..etc, and they are about 100 fps slower..
at all tempertures..anyone have a clue why that might be,,,,gfg

Every time you change a component or even change lot numbers of powder,primers,brass or bullets this can happen.As a result you have to retest and possibly adjust the load each time you change a component or even the lot number of powder,primers,brass or bullets.Are the cases ,powder and primers from the same lot?

The partitions and the ballistic tips are different bullets and would reasonably be expected to offer different resistance to their passage through the bore. This may arise from slightly different diameters, but you might not expect this with bullets from the same manufacturer. Perhaps the jackets are of differing hardnesses? What is likely is that due to the different profiles of these two bullets they have quite different bearing areas in contact with the bore. Pressure=force/area.
Different area=different pressure=different velocity.

Your barrel is telling you how it handles the bullets mentioned. If you are using a chronograph, don't form an opinion with one session, average several shooting sessions with the same loads, then draw your conclusion. A few shots does not an opinion make. Good shooting.

Nosler Partitions are stiffer, and harder to form to the barrel, than other bullets. They will ordinarily produce higher pressures, and higher muzzle velocities. Partition loads normally have to be reduced a little.

THANKS TO YOU ALL THAT POSTED