You may have figured your issue out by now but I might ask if the 3137 during load development and the 3198 were loaded with the same powder lot number. I have seen some variances with H4350 but not near that much....normally I'll see a maximum of 20 fps one way or another. As far as barrel speed up....after shooting for 55 years and the many benchrest barrels I've gone through the last 10 years from virtually all of the major manufacturers I will see a pickup of 50-75 fps with same load up to 100-150 rounds and everything settles in. With 6mm and 6.5mm mostly H4350/H4831 and 7mm using H1000 this has been my experience. Once I have brass fire formed, I then find the velocity it likes and after 100-150 rounds usually stays put.
On another note and nothing to do with OP, molybdenum disulfide does not cool the powder burn. Molybdenum disulfide as we know it for coating bullets is a layered structure with moly between layers of sulfide ions. It is sensitive to strain and can create hard flakes with extreme temperatures. Cycling high and low temperatures in a gun barrel can cause severe fouling for multiple reasons. When in the presence of very small amounts of H2O and oxygen the molecule becomes acidic and causes a reaction that produces molybdenum trioxide. This creates a potential for by-products that are abrasive as well as violent reactions with chlorine, sodium, and potassium compounds such as we have in smokeless powder. This compound is also not considered to be compatible with most metal compounds. The newly reacted compound can be detonated in the presence of heated magnesium and easily combines with various materials to form polymers. Tribologists and metallurgists studied in the art have a difficult time believing that molybdenum disulfide would be good for barrel life and especially where accuracy is concerned. With moly, each bullet is slightly different, each shot is slightly changing due to the above reactions and build-up...not counting the problem we already have with spent powder and copper fouling.
I have many friends who used it for years and switched back to naked bullets. They did not see an accuracy difference and actually did not see any difference in barrel life. They switched due to the cleaning issues. With that being said, you will never change a persons mind that has had success with moly. The same goes for those who do not use it. The last ten years of competition and shooting with hundreds of shooters, I have not seen a dozen folks who are still using moly.
