My SA WWII stainless still won't reliably feed JHP's. Ball ammo is perfect and no more FTE's after the break-in.
What I've tried: polished the ramp and even the chamber slightly. most every brand mag known to man, 18.lb wolfe spring. Over 400rds through it by now and it never gets through a mag of hp's without a stumble.
In the beginning it would fail to load them. You could see it was out of battery and you'd have to re-rack it, but with a new 18lb spring it now crams them in there and today they would fire, but not move the slide after firing. From the sound and feel I believe it was firing out of battery (ever so slightly). I'd have to man-handle the slide to extract the casing and start again... Frustrating.
My ammo was Fiochi, but I think some UMC got mixed in with my HP's so next time I'll try a different brand and be sure its the gun. Really seems like a tight chamber and maybe its the crappy UMC's that are causing it.