That .850" bullet length is what a couple of us on the Savage Forum arrived at through empirical testing a bunch of years ago now. I've made some file trim dies with which to shorten longer bullets which provided gooder and gooder results the shorter I went until I hit the .850" mark. Now, granted, I live at sea level and the results are slanted toward that elevation.
I just mic'ed a couple 87 Speer HotCor's and they're .820", give or take. I don't have any R-P 100 Coreloct's to measure, but since they seem to work ok in a lot of old Savages I would bet those stubby bullets are fairly close to .850". Keith Nystrom did a comprehensive chart of popular .25 bullets with calculations for stability based on currently accepted formulas. (The old Greenhill Formula is pretty much only valid for lower velocity stuff.) I'll send him a PM to get him to post it here. That chart of his can save a guy a lot of money and time if he's messing with a Savage 99 .250-3000.
My current .250-3000 is a Ruger #1A with a custom Douglas 1-10" barrel. Even though it'll nicely handle longer/heavier bullets, I still stick with the 87 Speers at 3000fps on the nose. Why? Because it shoots them very accurately, I have a metric sh*t ton of them, and they perform all the duties I'll ever subject the rifle to. Lots O'Deer have been killed over the last 100 years with 87's at 3000 fps.
