So there's this anime that is about children/teens fighting with little robots in their hands who shoot round bullets. Don't remember anything about the plot.
There are also toys like those robots used in the anime. I remember that there are extra parts to buy to upgrade the robot.
It's either to make it look cooler or just for more armor.
I'm surprised but I found the show I was looking for completely by accident. By typing "Densetsu" and looking for "Overture to a New War" on 9anime, I stumbled upon B-densetsu or Bdaman.
Battle B-Daman (Japanese: B-伝説! バトルビーダマン Hepburn: Bī-Densetsu! Batoru Bīdaman, "B-Legend! Battle B-Daman") is a manga series by Eiji Inuki which ran in CoroCoro Comics by Shogakukan from 2002 to 2005. An anime was created and it first aired in January 2004 in Japan, replacing Explosive Shoot Beyblade G Revolution in its 18:00 timeslot. It premiered in the United States in April 2005. Like its predecessor, it is themed around an enhanced version of a children's schoolyard game – whereas Beyblade was based around spinning tops, Battle B-Daman is a dramatized version of marbles. It is the first show of the B-Daman series to be dubbed in English. Toys were originally made by DD India and were released by Hasbro in United States. In Japan, Battle B-Daman: Fire Spirits! (B-伝説! バトルビーダマン 炎魂 Bī-Densetsu! Batoru Bīdaman) was superseded by Crash B-Daman, a new series with an all new cast and story. The original look of the B-Daman character was based on Bomberman (hence the name of the original version of the original toy: "Bomberman B-Tembak"), but the belt buckle is replaced with a "marble". The manga is published in English by Chuang Yi.
This sounds similar to Medabots Kids, teens, and adults could buy robots from the store, and have robot battles all over town. I believe they had "watches" on their wrists that they used to control the fights. This was on in the 2000's in the US.
The series begins with a ten-year-old boy named Ikki Tenryō, who wants to become a champion of the World Robattle Tournament. However, Ikki is unable to afford a Medabot, and his parents refuse to buy him one. However, he manages to get enough money to buy an outdated model, and, with a bit of luck, he finds a medal in a river. Ikki quickly inserts it into the Medabot he purchased named Metabee. The only problem is that the medal he found gives Metabee a severe attitude problem (a problem rarely seen in a Medabot), which leads Ikki to think he is defective. However, this theory is proven wrong later in the series, as it is revealed that Metabee actually has a "rare" medal.
It may be Little Battlers Experience, originally titled Danball Senki. From the Anime News Network description:
Danball Senki tells the story of a world where kids battle one another using little robot figures called LBX (Little Battler eXperience). Yamano Ban, a young boy who desires to become an LBX Fighter like his friends, is forbidden by his mother from owning an LBX as his father was taken from them in an accident relating to LBX years ago. One day a mysterious woman approaches Ban with a suitcase and tells him the hopes and fears of humanity lie inside it, and when Ban opens it, he finds a strange new LBX Robot. He is soon attacked by organizations who want the data inside this robot, and Ban and his friends are dragged into a corporation war that spreads all the way up to the highest political positions of power in Japan, and the true origins behind LBX Battling which could change the entire political structure of Japan.
Angelic Layer is about the right time-period, (1999-2000) and has the small robots that kids/teens fight with. But they used a thought-control rig to fight, not in their hands. Still, it's worth a look.
Eons ago, a cube–like peaceful planet named Quarton, populated by shape–shifting living bricks called Tenkai, was ravaged by a war between two opposite factions: the Corekai Soldiers, led by the valiant and fearless Commander Beagle ("Beag"); and the Corrupted Army, led by the ruthless Lord Vilius, who desired to obtain the source of Tenkai energy for his own purposes. However, the only ones who were strong enough to defeat him were the 4 Legendary Tenkai Knights of Quarton: Bravenwolf, Tributon, Valorn and Lydendor. Facing defeat, Vilius unleashed the mighty Tenkai Dragon, who was later defeated by the 4 Knights, its fragments scattered across the planet. These heroes were never seen again, but now, a year later, Lord Vilius and the infamous Corrupted Army have returned, more powerful than ever before.
Now, on planet Earth, in the year 2034, within Benham City, four young teenage boys Guren, Ceylan, Toxsa, and Chooki find an interdimensional portal to Quarton where they are chosen by Boreas of the Guardians to become the new generation of Tenkai Knights and prevent Lord Vilius and his Corrupted Army empire from taking control of both worlds
Tenkai Knights - Wikipedia
Do you maybe mean B-Daman Crossfire...????
B-Daman Crossfire follows Riki Ryugasaki, a young boy who eventually discovers a particular game called, B-Daman. One day, he finds a B-Daman of his own named, Thunder Dracyan. Thunder Dracyan is inhabited by a B-Animal, mystical beasts trapped in the form of B-Daman figures.
Riki soon discovers how Dracyan's B-Animal is one of the Legendary Dragons, specifically the "Blue Dragon". His friend and rival, Samuru Shigami also owns one of these Dragons, the "White Dragon" called Lightning Dravise. Another owner of a "Dragon" is Basara Kurochi, His "Dragon" is called "Twin Drazeros" The "Black Dragon" There is one more "Dragon" Force Dragren he is the red "Dragon" and his owner is Novu Moru. Together, they uncover a mystery and legend regarding these Dragons and plans by certain individuals seeking hegemony through them.
As a result, Riki and Samuru divulge themselves more into the sport of Cross Fight, learning the various new game types, mechanics and such while as they plan to thwart any B-Shot wishing to rule the world. At the same time, they meet friends and enemies in what climaxes in B-Daman Crossfire.
