Hommer

"Back to reality, I'll machine up some dummy bullets from brass, with different size meplat's to see how big I can go and still feed reliably from the FN's mag."

It's not just the meplat that affects feeding.

More so the angle of the bullet and the feed ramp angle
seems to have more / a fair bit to do with it.

Winchester Mod 70's in 458WM had a problem in that they would feed the 510gn winchester bullet but had problems with other brands.

It came down to having the feed ramp modified slightly on some guns so the combination of shoulder angle of the bullet where it curves and feed ramp works.

most problems with feeding bigger bores seem to be that it raises the angle of the bullet up too high so it jams into the back of the chamber instead of into the chamber.

That's just my HO.