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Not that complicated to figure out. A case that gets hot and under a lot of pressure fireforms to the shape of the chamber. Some spent cases show an imprint of reamer tool marks that formed the chamber. So a fluted chamber imprint is left on the case and a bottleneck cartridge may become a problem getting the case neck back to specs but some may reload them. The purpose of a fluted chamber is to aid in extraction by using the bypassing gases as lubrication. Also, the flutes in the chamber are not deep nor do they run the full length of the cartridge case...just deep enough to let gas flow down the flutes. I hope some are not confused.
Note; The fluted chamber part of my answer ends here. Past this point is just to further explain uses of fireforming.
A straight walled cartridge case is much easier to resize. On a bolt action some fire formed cases may be loaded single shot and timed to fit back in the chamber it was fired in. That`s a hassell up for debate but some do it by putting a timing mark on the case head of the first firing to match a mark on the chamber. Not likely on a magazine fed round with a semi-auto.
The P.O. Ackley improved chamber changes the body taper and shoulder angle to increase the powder capacity and also fireformed to be reloaded several times. Some may feel the need to time them as well. It`s just an edge to achieve a goal that many people in competition do to win. No swapping guns when this act is done because fireforming marries the chamber and case.
L.N.
Because the brass is weakened by the fluting when firing and would be prone to a case rupture if reloaded. Some people reload them anyway, but I'd leave them at the range.
Anonymous
Because of the fluting. Why is this not obvious?