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Well, as you say, the frames are in fact identical, and a complete .40 upper should be made to work reliably on any M43 or M40 frame. I have this combination working without issues (complete M43 pistol and additional complete M40 upper).

But you must note two things........... first of all, the recoil spring is NOT the same, being heavier the .40 one, check that if you don't want to damage the pistol.

Also, and this may be the clue, the slide stop is different. The one for the .40 is specific, and has more clearance for the bullet. Using a 9mm slide stop may cause the FTF you're getting. That's probably why the JHP ammo seems to work better for you than regular ball ammo. Are you using the same slide stop for both uppers???

Difficult cycling of complete live rounds is normal, it should work fine with just the empty cases.

Hope this helps.
 
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