I don't personally know of anyone in your area who can do it.
Wanna ruin it??? Take it to a machine shop.
Most good gunsmiths are good machinist.
Most good machinist are NOT qualified to be gunsmiths and many of them are absolutely terrible when they try it.
Bottom line there is a machinist just doesn't know what needs to be done when cutting a forcing cone.
As far as he knows a forcing cone is just a funnel at the rear of the barrel.
Cutting a forcing cone requires a special cutter head, a brass lapping head, and the critical forcing cone plug gage to gage the cone mouth.
Again, the forcing cone is absolutely critical and it MUST be cut with the aid of the plug gage.
You also have choices on the cutting head. You can cut a steeper or shallower angle cone for use with lead or jacketed bullets.
You have to buy the angle of cutter head you want.
Here's some possible help.
Midway has a listing a gunsmiths by area. Search it for someone in your area:
http://www.midwayusa.com/gunsmithlocator/home.aspx?utm_source=gunsmithlocator