I pocket carry a 649 when I'm on duty and the mim internals are starting to show spotty rust. Mostly in the annoyingly hard to reach spaces like the back of the trigger.

I read about hard chroming internals for increased resistance to the elements, plus the added smoothness of the chrome making the action feel smoother.

So my question is: What parts to chrome?

I know the hammer, trigger, and rebound slide. Am I missing another else?

I'm assuming wearable items like the hand and cylinder stop are not chromed

Personally I think you might open a can of worms up by hard chroming the internal parts. For one thing, your warranty would be out the door, but more importantly you also risk the possibility of the gun not functioning properly due to changing the dimensions of the parts by adding a plating. If you are a LEO the most important thing is to have your BUG 100% reliable!

I've carried Stainless M60's (internal parts are not all Stainless BTW) since 1980 and over the last 30 years, mostly in my pocket. They do get linty, damp from sweat and knocked around a little, and while I have had a few specs of rust inside from time to time, it easily and quickly comes off when cleaned. I routinely do a complete disassembly, cleaning and lubrication of my carry gun about once a year to make sure there is nothing detrimental going on. I've never had what I'd consider wide spread or serious rusting and the few little spots are easily removed. Like ANY piece of machinery, tool, instrument or mechanism a gun requires a certain amount of routine and preventative maintenance. I've yet to find any that are truly maintenance and 100% trouble free longterm.

As a rust preventative/lubricant I'd suggest using either Rig #2 Oil (MidwayUSA) or Remoil (anywhere). These two products are very thin, evaporate the carrier and leave protection and lube behind so they won't collect dust, crud, unburned powder & moisture which some thicker oils can do. Contaminated oils are not a good thing inside a gun. That is my opinion and others have their favorite products - but the main thing is to maintain and prevent rust from forming without doing further harm.

I do not own any guns that have MIM parts inside but I have seen a bunch that friends own. Truthfully, I've not yet seen any huge amounts of rusting and most of them are EDC guns.

I am not aware of others Chroming internal parts but I don't know what others do and never report.

You might want to consider NP3 or NP3 Plus by Robar
Firearm Finishes | Robar Guns
You can do the internal parts or the entire gun.

Back in the day I saw a few revolvers with gold-plated internals to reduce friction.

Only the bearing areas were plated so it wasn't noticeable unless you pulled the sideplate.

If you do this you won't want to plate the single action sear areas of the hammer and trigger or you could run into problems as mentioned by chief38.

Plate everything but the springs.
The hard chrome will actually make the parts last longer due to the very hard surface of the chrome, so maintenance intervals of parts like the cylinder stop and hand are extended indefinitely.

Hard chrome is not applied in a thick enough layer to affect anything except possibly an extremely tightly fitted full-on Match automatic slide and frame.
Hard chrome is typically applied in a layer around 0.004" average.
I've had a number of top end revolvers like Colt Pythons and S&W revolvers totally plated with NO affect on fit or function.
I always had customer any my own revolvers and auto hammers and sears plated too.
Never had any problems, including on heavily used police guns.

Hard chrome is totally unlike nickel or decorative (bumper) chrome. It's extremely thin and doesn't build up like other platings.

The operating tolerances inside a revolver are no where near as tight as most people think.
One area where you might think would be a problem is in the cylinder and ejector assembly being too tight after plating and causing the cylinder to stick or drag.
Never seen it even in Colt Pythons that are more tightly fitted then most other brands.

If it still bothers you, give Cogan a call. He apparently loves to talk about gun finishing and is possibly the best plater around.
They do plenty of revolvers with no problems.

Professional Gunsmith - Accurate Plating & Weaponry | Cogan Custom

This makes sense.

I did a lot of electroless nickel back when I 'smithed and I never had any problems with tolerances.

I used to have a lot of stuff Armoloyed. This is simply a proprietary version of industrial hard chrome. The thickness I was quoted for Armoloy was about 0.0001 inch. [They claim a bond with the base material at the molecular level. They cleaned up a piece of pipe and plated it and handed me the pipe and a large hammer. Despite severe metal movement, the plate didn't crack, chip or peel.] The thickness noted by dafariswheel appears to be missing a zero between the decimal point and the significant digit and even 0.0004 inch would be real thick. That's not gonna bother anything.

I did miss a digit.
In any case, hard chrome is very thin and won't have any affect on the fit parts.

Hard chrome on the surface acts like a case hardened part and causes the parts to wear much longer.
The "slick" chrome requires much less lubricant.
It actually improves triggers and actions due to the hard, slick surface.
Fouling won't adhere to the plating and is much easier to see and clean off. Often a simple wipe with a cloth is enough.
It's available in final finishes ranging from the original satin pearl-gray, to a brushed stainless look, to a full mirror finish.
However, the final finish has to be decided on before plating because once finished it can't be changed.

When hard chrome was first offered to the public a gun magazine did an article on Armoloy of Fort Worth.

The writer had a S&W "J" frame plated and did the following tests......

Put it on a table and covered it with damp salt for a day or two..... no damage.

Carried it for 30 days in a pants pocket with keys, pocket knife, and coins...... no marks or damage.

Tried to file it with a Swiss needle file...... the file only left a shiny line, no break though.

He had a common nail plated on one half and left the other end unplated. He then soaked the nail in a beaker of warm acid for several days....... the unplated end was almost totally eaten away. The plated end was untouched.

After that article I got a lot of requests to send in guns for plating.
I used Armoloy for years until I had a BAD experience with one of my own guns and switched to another processor.
The only reason I stopped using hard chrome for myself is that I replaced my guns with stainless steel when it started to become more common.

Armoloy of Fort Worth is one of the few that will also plate the inside of barrels and chambers.
Another top plater said that that only added "A little color" and in his opinion wasn't effective.

Hard chrome is sold under a number of trade names like Armoloy, SS Metalife, Metaloy, etc, but it's all hard chrome.