Well, if you were making your own gun from scratch, like Browning is doing, you could make the barrels as thick as you wanted. Also, nothing says the cone has to be cut with a reamer - it could be turned in a lathe to give any profile you like. In today's CNC world it would probably even be faster to turn the cone in the same setup as the boring as opposed to using a reamer.
My example is extreme for purposes of illustration but it serves to show that a longer forcing cone could well be less gradual than a shorter one.
