Yes ,I would try electroless nickle plating .....but the chemicals are expensive ,and the solutions dont last like electroplating solutions .....so unless you have a lot of nickle to do ,not economical ........Mention of hardchrome ....indeed a solution to the problem ,but the chrome deposit is intensely hard ,and sizing the chamber would be a very tedious job of internal grinding with a tiny wheel.....Diamond or CBN ,I d say.......Consequently ,sleeve the chamber with a bit of any HT steel you have .....an old gun may even have a soft iron barrel ,so any steel would be suitable ......Id have the lot done in one hour,ready to try out........There is no need of a reamer for these large cases ,just a small boring bar is good.