Here is the data from Hodgdon for 243.
100 gr SPR BTSP*__IMR 4350 __ Start 38.5gr____ Max 42.0gr __Vel 2958 fps*
I find it interesting that your barrel
Chronographed velocity was 3150 fps from a 22 inch M70 pre 64 FWT.
while the Hodgdon 24" barrel got substantially less velocity using the same load.