Interesting result with my .243

Same bullets, same charges and primers, just different cases.
100 grain Sierra Pro Hunters. 44 and 40 grains of H4831sc in Nosler and Winchester cases, lit with Remington 9 1/2 primers. 2.7 OAL.
With Winchester cases
40 grains 2850 fps
44 grains 3020 fps
With Nosler cases
40 grains 2950 fps
44 grains 3055 fps
I expected more of a difference with the heavier Nosler cases. As a side note. My deer load last year was a 95 grain SST over 44 grains of H4831sc in Winchester brass, Remington 9 1/2 primers OAL 2.635 gave a velocity of 2915 fps.
Curious indeed...

That's consistent with what I have seen. Nosler cases are thicker and have less internal volume so the equivalent load has less area to expand into when ignited and less case flex to absorb energy. Lapua would be a bit higher than the Norma.

Are these first firings on new brass or firings on already fire-formed brass?

The Winchesters are on their 10th firing and the Nosler their 6th. They're full length sized in Lee dies.

2,850 to 2,950 fps is no small change. I see 44 gr was much close,r but how many samples of each did you chrono to get the velocities ?

10 rounds of each. All charges weighed, not metered. Used an F1 Chrony, 15 ft from muzzle. Am amazed that with 4 grains less powder and a 5 grain heavier bullet, I can match my deer load in Win cases and beat it in Noslers. Am out of SST's at the moment, but will retest as soon as I get some.

Oops. Just about match it. Only lost 65 fps for a 4 grain drop in charge ( win cases).
I only shoot 10 rounds per rifle when i hit the range.
The 40 grain loads give more erratic velocities than the 44' s though. Shot to shots are upper 20's to low 40 fps. The 44' s go single to low 20 fps. Fouling does play a part though. A very clean barrel gives the widest variations and lowest velocities for the first 3 shots. Then things level out. Those Sierras are fast, but man do they lay down copper. The Hornadys leave hardly any at all.