^I can imagine that it would be very hard on the barrel.
There really isn't much reason anyone would want to short of just seeing what would happen.
Going in the other direction, someone on this forum built what he calls the 12GA rifle from hell. It is a .50 BMG cartridge that has been necked up toa straight walled case and loaded with 12 gauge slug, which makes it something like a .70 caliber.
http://thefiringline.com/forums/showthread.php?t=197214