This is Careful's personal prototype VSR-X that he built and documented on his forum thread here years ago.

I will include what I have onhand for the 0.69g BBs this was designed to shoot with, and I'm happy to hook you up with more via my company. It is an absolute monster of a sniper rifle. Happy to answer any questions you may have to the best of my ability.

It's currently set up shooting ~273 FPS / 2.38j with the 0.69s, which is field legal where I play. It's capable of 5j plus with the correct spring, and that's how Careful ran it when he made his record shots.

I'm happy to answer any questions to the best of my ability.

I have tons of correspondence with Careful to verify the legitimacy of its history.
I'll ship on my dime anywhere in the CONUS if you meet my asking price.