Any gunsmith can do it and any machinist worth his pay can do it.
The thing you will run into besides the obvious metal finish is barrel harmonics.
Removing material relives stress. I'm not saying it WILL happen but it most certainly CAN happen; You may end up with a warped barrel. That is highly unlikely but it will change the frequency that the barrel vibrates at and if you thin it out enough, with a barrel that long, you could get barrel whip.
Even if none of that happens, do not be surprised if loads known to be accurate now are NOT accurate after thinning it d