First off from reading your post, assume you do not have a "machine rest". When testing loads like you are doing to really take the human error out of it & this is very valuable tool to have. Not having that, my humble opinion if you can opt for the 1:14 bbl. Stick with the .356 diameter bullet. 2.9 Vit 310 in my opinion little hot with a HBWC, but a hard cast bullet should work fine. Try dropping the charge to 2.6 and see what happens at the 50.
How deep are you seating the bullets? what crimp? Refer back to older posts from others for their suggestions. Make sure all the cyls ranges to bbl center bore when it locks up to fire. Do you have a range rod setup? With the 686 try some 158 grain SWC's too.
Big factor is quality of the bbl you have on the gun. I have had guns I thought should have shot great and didn't because the bbl was poor quality. Is it a first class air gauged bore with good lands and groves. Makes a difference for a competition gun. W
When you slugged it, did bullet go thru smoothly or "jump" especially at the area that screws into the frame?
Just some thoughts.