Zel Custom Manufacturing recently purchased the Ultralite50 line of .50 BMG AR-15 Uppers. They have just released an improved version of the Ultralite50 called the Tactilite.
As you can see above the Taclite is a single shot bolt action that sits atop an AR-15 lower. Because the lower is considered a firearm by the BATFE, the Tactilite is legally an accessory not a firearm, a fact that I am sure would freak out certain elements of society if they ever found out!
It features a free-floating barrel and is available in barrel lengths 18.5″, 22″, 29″ or custom lengths up to 30″. There are three main models. The Ultralite is the budget model that has a Mossberg barrel. The Duty is next step up and is the basic law enforcement / military model. It features a Lothar-Walther barrel. The Super Match is the competition-grade model featuring a Lothar-Walther Super Match barrel.
They also sell a variety of packages. One of the packages they sell is the ultra-short 18.5″ barrel UltraLite Defender. It is designed to be fired from the shoulder (as opposed to a bi-pod) and is packaged with a set of back-up iron sights (BUIS) and sling. I don’t think a single-shot .50 BMG is ever a good choice for self defense, as the name suggests, but it could be popular with hunters wanting to carry something exotic during their next hunt. While it would have the power for African dangerous game, civilian ownership of military cartridges are banned in many African countries where hunting is popular.
Pricing starts at $1550 for the most basic model.