The chemical composition of the two powders is exactly the same. fffg just has smaller granules. Therefore for a given volume, more of fffg will fit, so it will weigh more and therefore be more powerful. I use it in my sidelock because it is easier for the smaller granule to fall into the sidelock chamber than the bigger granule. Just remember as nchawkeye stated, it is more powerful, so I reduce my volume about 10-15 % to achieve similar results. In other words, 85-90 gr of fffg is roughly equivalent to 100 gr of ffg in power.