jakes10mm: I have used H110 and WW296 alternatively under 240, 295 and 300 gn 44 mag slugs, and have varied my powder charges, without ill effect. For the heavier slugs I will load between 19.5 and 21.5 of H110 and just as long as you have a good crimp on the bullet you shouldn't suffer any ill effect.
I don't know where the heck that cautionary note came from, but I know Winchester states flat-out - don't use reduced loads with WW296 - I think they may have been afraid of target or squib loads and a light crimp that maight give poor results but, H110 and WW296 are the same powder and interchanging loads for either powder hasn't hurt either of my M29s at all. And, the accuracy is great. HTH. Mikey.