You know Larry, I can't build you a rifle if you don't visit the P.O. once in a while
Everything on earth has an RPM threshold. I know this very well from dealing with air bearings. There is a point where things fly apart no matter what they are made of. Air bearings are a very curious thing. Especially the ones that are used in the dentist office. Those drills spin at 400,000 RPM and that is no little feat. I have tried to make an air bearing from scratch because I was fascinated with the way it functions, because the internal parts ride on a cushion of air, which allows nearly effortless motion.
So I set about to build an air bearing. I precision ground all the parts, and worked for weeks on it. I finally got it to float, but it would support no weight, and would crash if it was spun. Considering that, the dental tools that are found in every single dentistry in the nation, functions in a very narrow limit of parameters. There is nothing easy about it! One fleck of dust, one ten thousandth of an inch in the wrong place, and it wont even try to work, yet they are found in every town in the nation.
The point is, we are surrounded by much more difficult things than shooting a lead boolit at high speed through a precision barrel. I really think that if you posed this question to a bunch Boeing engineers and scientists, and made it their mission in life to make a cast lead boolit fly as fast as possible while maintaining accuracy, they would probably deliver 3000 FPS out of a 10 twist with 1/2 MOA accuracy. If you knew the hoops they jumped through to bring you such novel inventions as the jet engine, you would be duly impressed.
The problem is, we're loading janky boolits, in janky brass, shooting them in janky barrels, that are screwed into janky rifles, and wondering why we can't get them faster.
Personally, I think a lack of precision across the board is jerking the rug out from under us every time. It really is like trying to sign your name with a fountain pen that has been duct taped to the end of a broom handle.
I see the RPM threshold as a measure of boolit perfection. I mean, if you could get a perfect boolit out of the dad-burned barrel, there's really no reason why it can't be pushed way up there in speed. If the boolit was perfect, and unmolested, I would think that the true RPM threshold of water dropped WW alloy would be about 3900 fps. At that point, the lead would literally fly apart from the g-forces imparted on it.