I'm not an expert but in my limited experience, I've seen it done with sand blast, chemically (see Brownells) and reverse electro-plating.
Not that I'd recommend it as the preferred method but the sand blast did surprisingly well. I didn't use a blast cabinet, etc. just a blast gun (less than $20 at Harbor Freight) and a bag of fine masonry sand (less than $10 at most any building supply store) and did it outside in good sunlight. You do need heavy leather gloves (I used high-cuff welding), long sleeve shirt, mask or respirator (you don't want to breathe the mica dust) and face protection along with an adequate size compressor. I also wore a sock hat as the media will get everywhere. A regular sand blast hood would work, especially if going to do much blasting. I did plug the bore and cylinders and used caution on certain areas. I didn't do it but if you wanted to recycle the media, put a good size plastic tarp down and catch it. It seemed to work well on small jobs and was the cheapest method for me. Unless you already have access to a compressor, you may want to do the chemical. If you decide on electroplate, IMO, bumper shops do good on big parts but aren't so good dealing with smaller fitted parts for guns. They may remove the plating though without problems.
ETA:
There are numerous places that re-blue and can probably also remove the nickel. Fords comes to mind for bluing, but they may be backlogged on jobs.