If you want steel case like 8m3 or SSTs to expand, min. 12." 154gr sp will not expand under 16"
If you have a can and just want a blaster, 9.5 is ok. It'll cycle the subsonic steel case on setting 7 (the 200gr stuff that isn't supposed to cycle semi auto). If you don't have a can, the muzzle blast is kind of ridiculous.