I was asked this question some years ago when they became the "in thing" in the UK. I listened to both sides of the argument,I spoke to shooters and to the Proof house who had carried out some unofficial tests and came to a simple conclusion. I dont care what any one say's but at 35 a time I'd do them all day .
I have my own reamers, hones, etc., for several gauges to lengthen cones. I've tested enough to satisfy myself that patterns get better with a longer cone. This seems to be more dramatic in the little .410. I attribute the pattern improvement to less damaged shot from abrupt changes in the shot path. Since more percentage of the shot load is in contact with the barrel wall with a .410, I think it reflects in the more dramatic improvement. I use a reamer that produces a 3" long cone in a .410. I've tested my barrels enough that I'm satisfied.
