The smaller shaft sizes will penetrate better than the larger ones, but the penetration is not noticable. The smaller shafts, the SST carbon shafts, would actually shoot slower than most carbons.The reason for this is their weight. They are heavy shafts for carbon. The few extra grains you get will only slow your bow down by one to two fps (the general rule is for every three grains you gain, you lose one fps).