Chrome lined rifle, machine gun, and shotgun barrels have bores that are indeed chrome plated. The process electroplates a very thin plating of chrome on the inside surface that is done for wear resistance, and in the case of rifles, inhibits erosion from hot powder gasses during rapid firing. The very smooth surface also makes cleaning and maintenance easier.
The process for the bores of firearm barrels is specialized, and much different from the process that was used for decades to chrome plate bumpers and automotive trim. That was a multi-step process that involved an extremely thin electroplating of copper, followed by a much thicker layer of nickel, and then followed by a very thin layer of chrome. When this process was not done perfectly, sometimes the chrome would begin to delaminate, flake, and peel off. The exterior of some handguns were also plated this way, and we often see older ones that have the chrome flaking off.
https://faxonfirearms.com/blog/frequenty-asked-questions-nitride-vs-chrome-lined/The chrome is not alloyed into the barrel steel. It is only deposited onto a surface that has been etched and prepared to ensure very good adhesion with the steel. However, there are alloys such as 4140 Chrome-Moly steel that contain roughly 0.1% chromium, along with molybdenum, and manganese that provide more toughness, corrosion, and abrasion resistance than the earlier low carbon barrel steels found in our vintage guns. Opening the chokes on a Chrome-Moly barrel will not remove the chrome. It is indeed alloyed into the steel.
Many gunsmiths charge more to open chokes of chrome lined barrels simply because tooling such as the carbide reamers or cutters mentioned by Dr. Wanker are not used by most gunsmiths that do a lot of this work. Instead, they prefer to use something like a rigid Sunnen sizing hone. Also, the chrome plating, although very thin, is much more than "a couple molecules thick". And it simply takes a bit longer to properly get through the harder surface deposit of the chrome than it would to open the chokes by an equivalent amount on a vintage shotgun barrel with mild steel barrels. Time is money, and most gunsmiths charge for their time. Some may charge more, or even refuse to do the job, based upon myths and misconceptions about the job. When the surface layer is removed from your chrome lined chokes, the new polished surface in that area will be whatever alloy of steel the barrel was made from. Since the chrome lining is seldom much over .0015" thick, opening the chokes is going to completely remove it in that area. It is unlikely to be very evident if done correctly. But over time, the bare steel could begin to oxidize or show the effects of corrosion while the rest of the bore maintains its' chrome plated surface.
