Does your false bottom sit right at the curve for the bottom of the keggle? I have a Coors keg keggle, which might be slightly different than a BM keg, but about .5-.75 gallons of liquid fits under the false bottom, so you figure the volume of the keg - the false bottom is somewhere between 14.75 - 15 gallons. How much grain you could fit from there would depend on how much water you intend on using. You could fit a good bit of grain (30-32 lbs or so) if you were doing a high OG 5-6 gallon batch (like a Barleywine), or fit a moderate amount of grain (less than 25 lbs) if you were doing a 7-9, maybe 10 gallon lower OG batch (like a session ale).