I have an early 1960's Model 41 frame with a recently purchased 1980's 7" barrel with a Red Dot. It's a great Bulls Eye set up. However, about once every 25 rounds of CCI Standard Velocity ammo, it misfires. The round chambers, the slide is fully forward, the trigger is normal and the hammer falls. The rim of the misfire cartridge gets a light strike, or no strike. I have tried different slide springs from 6.0 to 7.5lbs, lighter & heavier extractor springs, greasing the extractor ramp, and cleaning everything extensively. The original 5.5" heavy barrel does not exhibit this behavior. I am out of ideas here. Any help appreciated.