I would presume that for an accurate calculation of this base area for pressure one would need the inside diameter of the case at its base. Not wanting to try & determine that I simply made two quick calculations, one based on the two bore sizes, the other on the OD of the shell just ahead of the rim. In both cases area of the .410 is approximately 45% that of the 20ga.
45% of 12,500 = 5,625. I would not foresee any problem with total back thrust using the .410 in a 20 ga. That max pressure is going to be realized in the chamber itself where it has the additional strength of the inset, some at least which are quite capable of containing the pressure by them selves. Pressure should drop dramatically as you exit the insert into the bore of the 20ga as its area is 225% that of a .410 bore.
It would appear to me the only area of real concern would be that possibility of a Thin area in the breech face R'Man mentioned, but thats not real likely.
