Both...Over 50% of black powder is left as residue, some blows out the barrel and some is left in...Since the grains in FFF are smaller, they burn faster and more complete, since it takes less to reach the same velocities less fouling is left in the bore...

I used FF Goex in my .54 for a few years back when many thought the standard was FFF for .45 and under and FF for .50 and over...We now know this isn't true...I switched to 80grs FFF Goex in my .54 flinter years ago...I also use it in my inline and as priming for my flinters...