I assume you had a good recipe for making black powder. The key word being "good". The problem is not with the large granules, it is with the small granules. The smaller the granules, the faster it burns and produces higher breach pressures. The coarse granules will ignite and burn slower and, thus, produce less breach pressure. Lower breach pressure translates to it being safer than the fine stuff. Burn rates are relative and the differences may not be perceptible but it is there and it makes a difference. Bigger grains = slower burn rate = lower breach pressures. Smaller grains = faster burn rate = higher breach pressures. Since I have no idea what recipe you used to make your black powder, I can only say that what I have said about burn rates is only relative to varying grain sizes within the same recipe and batch of powder. As with any experiment using black powder, especially home made black powder, there are three important rules. First, proceed with caution until you know what your powder will do. Second, proceed with caution. And finally, proceed with caution. 'Nuff said.