Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2·H2O) which reacts with the metal ions to deposit metal. The alloys with different percentage of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.
This technique did not come along until the mid 1800s, so electroplating was already being done. Bright nickel requires a DC power supply.
Electroplating nickel usually gets a thin layer of copper (called a strike) on the steel first. Then the nickel goes on. One problem that can come up is the copper layer can be damaged when using cleaners that contain ammonia. Then the nickel plating can come loose.
And again an Internet myth is perpetuated!
Most, if not all, gun makers do not underplate with copper. It is not necessary.
Electricity use has been around for a long time. Much longer than most people think.
Rudimentary plating can be accomplished by simply mixing the proper solution and immersing the item.