I'm no expert, but it is my understanding the LW poly barrels are cut with a different button that is more rounded, and does not cut the sharp and deeper grooves found in traditional cut rifled barrels. They are not like the traditional polygonal barrels that are hammer forged, which requires tremendously expensive equipment. If my understanding is correct, less metal is removed from the bore. This would presumably result in a weight difference, but I would be surprised if it were as great as 15%. If dimensions are the same, I would guess a different grade of steel would be necessary to affect weight that much.