Try 43-46 gr of RL-22.

I use 46 gr of RL-22 with a remington corelokt bullet- its very accurate with this load. I don't have a chronograph, so I couldn't tell you the velocity, but in comparison to american made factory loads, the load I use has alot of bark and a bit more recoil.

I've only reloaded 140 gr. bullets so far as I bought a lot of 1000 corelokts a few years ago and want to use these up first, I've also tried 140 gr SMKs and gotten better accuracy.

From everything I've read about the 6.5x55, it seems to prefer slower burning powders like 4350, 4198, and RL-22