Depends on the type of nickel you have too. Modern electroless nickel plating is by far one of the most durable finishes one can have done. It is very popular for industrial usage as well. Electrolytic nickel plating however is not quite as durable, and is a rather old process. It would help if we had a picture of the gun in question to determine which it is, but as a quick way to tell, if it's shiny and smooth it is likely to be electrolytic whereas electroless tends to match the material on which it is applied (IE the same pits and blemishes in the metal carry through) and it is more coarse than it is smooth.

You said it is a matte nickel finish, so I would be inclined to say it is the more durable electroless nickel finish. To attest to how well this holds up, I have the hammers on two pistols of mine finished with it.

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Wheelguns 'n 3rd Gens.