FWIW, the starting loads shown in Lyman 49th Ed. is 2,900+ fps, MV. Not exactly a slow load. Can't go slower (less powder) w/o possible danger.
Go to the Hodgdon website, and try looking up a load using Trail Boss powder. It's a bulky pistol powder, mostly used for low velocity "game" loads by CAS shooters, but works well for very low velocity centerfire rifle cartridges sometimes.
I remember seeing some load data there for 100 grain .243 bullets pushed to some 1200-1800 fps, or thereabouts.
Should be just the ticket for shooting turkeys with a .243.
Edited to add: I just checked the sight, and the data shows loads for Trail Boss with velocities from 1045 fps to 1603 fps. Would think this data should work great for a turkey load in a .243.