I've had the same thoughts. The calculations that we use for backthrust are flawed at best as there is no allowance for going from a shotgun hull(straight) to your typical rifle cart.(tapered). Daryl has it right when he says pressures must still be in line for the parent cartridge. You are are correct in that a straightwall will be easier on the action. Ellis is correct when he states that even when the math has been done one must still proof a double.