Larry, you are on the right track, but I'd like to add a bit of information. There are two issues of backthrust. Should the "face" be a bit thin, esp. around the firing pin, it could deform under the higher local pressure of the .410. The other issue is total backthrust. That needs to be calculated using the .410's pressure times its base area and comparing to the same calculation for the 20 bore.


DDA