Not Merkels, but in all my various calibre doubles (even Nitro Express) I use cast bullets on most occasions. The price and availability of jacketed bullets makes cast a real alternative.
My thoughts:
1) Cast bullet at or close to proper regulating weight
2) cast and sized ~.001” over groove-groove diameter
3) Nitro guns I use “Lyman #2 alloy”
4) Gas check for sure
5) for Nitro velocities use “LBT Blue Lube” if you can get it.

No idea about powder coating, have never tried it, but hear good things.

- Mike