WARNING: The following contains spoilers for Chapter #65 of Boruto: Naruto Next Generations, “Karma Power,” by Masashi Kishimoto, Mikio Ikemoto, Mari Morimoto and Snir Aharon, now available in English from Viz Media.
In the Naruto series, the Rasengan has been the titular character's most powerful weapon. This blast of energy, akin to a one-handed Hadoken from the Street Fighter franchise, has been used in battle to defeat the Akatsuki as well as villains from the Uchiha clan, like Madara and Obito, and powerful beings such as Ōtsutsuki Isshiki and Kaguya -- making it a standout in pop culture. The move has even crossed over into music and MMA fights, with the likes of Izzy Adesanya using it to taunt opponents in the UFC.
The Rasengan is a spinning ball of chakra formed and held in the palm of the user's hand, which is only known to a handful of shinobi. It was created by Naruto's dad, Minato, in response to the Tailed Beast energy ball, and does not requiring any hand seals. The user can smash it into opponents like a punch, or, as the Hokage and Boruto have shown, they can apply wind techniques to turn it into a throwable ball. It can also be molded into a shuriken, with Naruto and Boruto surpassing Kakashi and Jiraiya by having shadow clones using the move too. Even Sasuke admitted it's more powerful than his Chidori lightning blade, when Naruto and Konohamaru turned the Giant Rasengan into big bombs.
In the latest Boruto chapter, Momoshiki drastically altered this already powerful technique as he possessed Boruto once again. He wasn't holding back with the boy's Karma mark, using Giant Rasengan to try to kill Code because he wanted to kidnap Kawaki to feed to the Ten Tails and obtain the chakra fruit. However, when Code took Kawaki hostage, taunting Momoshiki that he couldn't fire the bomb off as it'd kill Kawaki, Momoshiki proved he's as crafty as they come.
He turned the Rasengan into a Rasendan, with the manga's annotation noting "dan" means "bullet." He aimed his fingers like a gun, condensed the ball into a pellet and sniped out Code. He shot through Kawaki's underarm and blasted Code in the arm, letting him know he was toying with the ex-Kara agent.
This development reminded fans how adaptable Momoshiki is, and that he can appropriate Boruto's power at will. The scene also implicitly referenced how Boruto worked with Sasuke, applying lightning to create a Vanishing Rasengan, which means Momoshiki might be able to use invisible bullets soon. Ultimately, it left Eida petrified as she doesn't seem able to predict what the alien will do next in Boruto's body. Kawaki's also scared because this gave Momoshiki the opening to kick Code around and take the teen for himself.
Shockingly, while Naruto entered the war to save Boruto and Kawaki, things aren't looking optimistic for him; though he has his old Rasengan, he doesn't have Kurama to power it up. As such, Momoshiki is the one that has the WMD everyone is afraid of, leaving them wondering what other upgrade he'll apply next.