I think there are 2 separate forces involved, the breech thrust (acting to the aft, or toward the shooter) is typically what can fail the bolt lugs in a bolt action rifle. In a brake action rifle, this force is on the flat face of the receiver. The forward thrust on the barrel is the critical load for the brake action and will cause it to loosen. Then the over pressure can split a thin barrel if the chamber has been enlarged too much.
I believe the aft thrust and the forward thrust acting ON THE GUN, are not equal. Some of the forward thrust acts on the bullet, some but not all of this is then transferred to the barrel via friction and makes up the total forward load on the gun.
This is why I think you can get by with more pressure on a straight or slightly necked case. Remember the tennis ball shooter made out of a tennis ball tube, not much structural integrity between the tube and the end cap, but it held for a while if you place it flat on the ground.