Hejsan bendtsen! I was about to build a double 500NE last winter and did some calculations on actions strengths. The particular action was an ower/under in cal 12/89 with the additional Kersten locking. With classical Nitro Express loads it should be sufficient, but I have not found the working pressure of the super magnum 12/86 shots. The idea was to cut the barrels of at some 3" and fit the rifled barrels to the action by fitting them into the shot chambers (or something). It might have been too much of Fun to get it to regulate considering the bullet prices, cooling of barrles between shots etc... I�m glad it did not come to fruition.
The thrust of the case against the action can be calculated by dividing the pressure in the case with the internal case head area. Since rifle cartridge heads are usually a lot smaller than the shotgun shells they can be loaded to correspondingly higher pressures. But the large and wonderful NE cases are close to the upper limit, with 577 and 600NE far too large. But e.g. the smaller 9,3x74R are very well adapted to this config. That case could be wildcatted to a 416x74R that would be rather close to as powerful as a 416 Taylor.
There is some guy in Bj�rneborg(?) that apparently has done these conversions. If you like to enjoy the art of speaking finnish I could get you his tel number.