I did small parts with a Brush Plater and it worked fine. Texas Platers was the brand but there are others now I believe. I used that for about 35yrs or more and still do occasionally. Does a great job for that,,triggers, screws, pins, levers, safetys, etc.
For complete Electro-plating jobs on the engraving projects I had always used Reliable Plating in Chartley,MA.
They had been around for nearly 100yrs and did a perfect job. They had an FFL and were familar w/firearms. You did all the polishing. Turnaround was usually 2 weeks.
But they are gone now having been recently sold to another plater in RI.
I don't have any experience with the new outfit (Monarch?) nor any feedback from anyone in the biz that has used them.
Lots of engravers used Reliable Plating over the years. Don't know where their business is going.
As far as a DIY Electro Plating project, it is doable, but expensive.
All the nickel plate soln(s) can be bought on the Net
as well as the necessary power source and hook ups.
Some sell 'kits' but many are set up to do the plating in 5gal plastic buckets (that you pay dearly for). A bbl'd action (rifle) certainly won't fit in there.
You have to know what you are doing when it comes to the power/current and how much to apply in the plating process or you can end up with some bad results.
Everything For Firearms - Electroplating Kits - Caswell Inc
You can get adventurous and make both your own Nickel Plating Soln and your power source from common items available!
Again internet instructions available but I'd practice on a scrap piece before immersing my pride and joy firearm expecting a brilliant nickel plate.
Here's one such set up. I have no idea if this works or not. Just showing you what some folks are doing. Everything 'works' on the Internet!
High Quality (and Safe) Nickel Plating : 7 Steps (with Pictures) - Instructables
Yes prep is very important. Perfect polish is king, and so is cleanliness the parts. Just like in Bluing.
Handling the plating soln correctly, the time in soln, agitation is sometimes needed, etc.
You might want to look into 'Electroless' Nickel Plating.
A different way to a Nickel Plated a surface. Still not in-expensive to get set up for. But can said to be geared more to the DIY'r than the Electro-Plating set up.
Good Luck with your project!