Well that would take some annealing... *Just* necked down? Given the wall thickness of the neck at full size, isn't the final wall thickness a bit thin?
Imagine how long the barrel would need to be to burn all that powder fully.
5100 FPS you're getting into the kind of speeds where stability becomes an issue because you can't spin the bullet fast enough to gyroscopically stabilise it without fear of it breaking up.
I mean if they've solved all those problems which I'm sure any good engineer can do then you're probably looking at a 5-600 yard bunny calibre.
That is the most American thing I have ever seen
I know it's an April Fools joke but I bet somewhere in Texas there is a bloke going that would work.
The bullet would make Star Trek going to warp noises as you fired it