I purchased a used loaded Springfield Target 9mm with what appears to be an STI Squared Match hammer. The ignition set was very polished and the trigger pull is a wonderful 2.75 to 3 lbs for USPSA competition. I tried a 17 lb mainspring with frequent light hits on CCI small pistol magnum primers (easier to get). I am now on a 19 lb spring which improved, but didn't cure, occasional light hits. I have a 20 lb. ordered, but wonder if I am overlooking part of the equation. I run a 19 lb in my Sig 1911 target that has a Wilson Combat hammer that never fails to ignite standard large pistol primers. Are the magnum primers that much harder or do I just need to keep increasing the mainspring weight until I get reliability? Both pistols have a Smith Alexander housing magwell and the strut is the same length as my Colt 9mm.