I 'm trying to understand a difference in the accuracy between my 308 bolt rifle using a 215gr cast GC PC bullet & the 260gr GC PC bullet in my Marlin 38-55. I drive the 215gr 308 ( sized .311 ) to 1763 fps with about 1" groups at 100yd. In my Marlin ( Deep Grove Ballard rifling ) 38-55 with a LBT 260gr GC ( sized .380 ) I can't exceed 1485 fps without accuracy going to 3" to 4" at 100yds. Both bullets are using the same BHN 15 alloy. What am I not considering. I thought I would be able to easily get 1600 fps in the 38-55.