I'm loading 44 mags for a Ruger SBH and I'm using a Lee 310 gr cast bullet. It is of the long wide nose configuration and has two crimp grooves allowing for a long OAL. The powder I'm using is WC-820 and the powder came with info saying to use AA-9 loading data but it seems to be more closely related to H110 instead of AA-9 so I normally just go with a grain below max for H110 as the max for the WC-820. This has worked for other bullets it seems other than this one. I'm getting sooty brass as if the brass isn't sealing off due to low pressure but I'm using 20 grs. of the Wc-820 behind the 300 gr cast bullet. I am taking advantage of the lower crimp groove so the OAL is pretty long (nose of bullet is almost flush with end of the chamber) but I would think that it should have enough pressure for the brass to be sealing off. The primers still look fine and the load chronographs at 1275 fps. What do you guys think is going on here ?

I'd take a guess that the pressure curve is low on the front end of bullet travel and not sealing the case until gas gets between the case and chamber wall. Other than being messy, its probably not an issue if you like everything else. You don't mention what primer you are using, but you may want to switch to magnum primers if you are not using them and see what that does. (Back off slightly on your powder charge, magnums are about .5g-1gr hotter than regular ones. Kudude

I using Winchester primers that says on the box that they are for standard or magnum loads.