What are you going to kill using this?
…also, in that image, the .50 is the small one. The big one is a 20mm, using the same ammunition as the M61 20mm autocannon the US Air Force mounts in fighter aircraft, and those are not for sale to the public in the US. Though the manufacturer, Anzio Ironworks, has developed a proprietary cartridge for the same design, the .50 Anzio. It’s that same 20x102mm casing, but necked down to accept a .50 caliber bullet. Theoretically it is permissible for American civilians to purchase it. I don’t know how many they’ve sold. I believe they are regarded as “long-range target rifles.”
It’s very powerful, but not really comparable to the 20mm, and not as ballistically efficient. This is unavoidable. Energy is the capacity do work and in a firearm the expanding gases do work by exerting force on the base of the bullet, accelerating it down the barrel. The 20mm has two and a half times the bore area the .50 does. The .50 Anzio can get a 661 grain steel core armor piercing bullet up to a bit over 4000 feet per second, but that velocity requires a barrel that’s 72 inches from breechface to muzzle. It’s a bit more of a crew-served weapon than a rifle in the conventional sense. They make a “lightweight” version with a 50” barrel, but it gives up about 10% of the velocity the 72” barrel generates.
I’m not even sure what kind of optical sight is suitable for it. How much magnification is needed to take advantage of its flat trajectory and accuracy at range? 20x? More? Does it need a rangefinding reticle, like a mildot setup? Or would any sensible end user already bring his own laser rangefinder, and perhaps a thermal sight? Yes, you can buy thermal weapon sights in almost all of the US. They’re extremely expensive, but maybe not to the person who buys that kind of rifle.