The Fortnite v17.10 update added new Alien Nanites to the game. The primary use of these Nanites is to give players the ability to craft alien weapons.
Now, players can create Ray Guns out of SMGs, Pulse Rifles out of ARs, and Rail Guns out of Snipers.
The problem here is that the overpowered Rail Gun has been removed from competitive play. Nanites are in competitive, though, allowing players to acquire a Rail Gun through crafting.
This is a clear oversight by the Fortnite team, and will probably be adjusted in the coming days.
We may even see Epic remove Nanites from competitive while they work on a fix for the unintentional inclusion of Rail Guns in Arena.
Nanites are a great way for players to get the loadout they want. Some players prefer Pulse Rifles for their versatility and accuracy, and won't need to rely on RNG to get them.
Unfortunately, this oversight means that players will also be one-shotting you through a wall in Arena.
If you can't beat em, join em. You might as well abuse this feature until Epic patch it out of the game.