Agree with Small Bore. Bell's tests report some reduction in pressure when cones are lengthened. But like SB, I doubt I'd bother unless the cones are short and steeply angled--which they sometimes are, on older guns.

Brister reported a reduction in felt recoil (with no velocity loss) on a gun that had both steep cones and was underbored, after barrel "surgery". However, he not only had the cones done but also had it overbored, so it's hard to tell what caused the change.