Swift A-Frames don't hold together because they're "stiff." Instead they hold together because the front core (which is pure lead, an hence pretty soft) is bonded to the jacket, and the jacket has a partition between the front and rear cores to stop expansion. Consequently they'll expand just fine at the slightly lower velocities from your 20" barrel.
This is also true of other lead-core bullets, whether Nosler Partitions, North Forks, Trophy Bondeds, etc. etc.
