I shoot the same MK-85 with the 209 conversion. I can't believe anyone else is still shooting this old classic modern ML.
Because this gun was designed and built when 100gr of BP was considered a maximum load, I would limit my testing to that level. However, I have used 100gr charges of 777 extensively, with bullets of all weights up to 348gr, and have experienced no ill effects.
Personally, I like the 270gr-295gr Powerbelts which are easy to load and give good accuracy in my rifle. The 270gr Plantinum bullets have an improved shape and a higher B/C if that does anything for you. They shoot at just over 1,700 ft/sec when fired over 100gr of 777. I've also used the tried & true Hornady XTP's which also shoot well but they are a pain in the neck to load and the B/C is terrible. They do, however, leave a NASTY hole in a whitetail.