Such rifles are huge, heavy, expensive, and difficult to transport. I can't find a single instance of a .50 BMG rifle being used to murder someone in the US. Anti-gun groups like Violence Policy Center have one report of a .50 caliber rifle being used in a homicide[1], but Adam Wickizer was a convicted felon and was diagnosed with bipolar disorder.[2] That double-disqualifies him from possessing firearms. He most likely used a .50 caliber muzzle-loader black powder rifle, which is far less powerful and does not require a background check to acquire.
If you want to restrict things based on how often they're used in crimes, you probably want to go after handguns. Around half of murders are committed with handguns.[3] Rifles of any kind are used in ~3% of homicides. For comparison, twice as many people are murdered with bare hands than with rifles. And despite civilians owning .50 BMGs for over a century, nobody can find an instance of someone in the US being murdered with one. In short, the data does not support restricting these things.
If you want to restrict things based on how often they're used in crimes, you probably want to go after handguns. Around half of murders are committed with handguns.[3] Rifles of any kind are used in ~3% of homicides. For comparison, twice as many people are murdered with bare hands than with rifles. And despite civilians owning .50 BMGs for over a century, nobody can find an instance of someone in the US being murdered with one. In short, the data does not support restricting these things.
1. https://vpc.org/regulating-the-gun-industry/criminal-use-of-...
2. https://www.wnep.com/article/news/local/lackawanna-county/fa...
3. https://ucr.fbi.gov/crime-in-the-u.s/2019/crime-in-the-u.s.-...