Tokyo Marui is very well known for being one of the highest quality manufacturers when it comes to airsoft guns. They are one of the longest lasting airsoft gun companies that still produce airsoft guns today, they were established in 1965 and produced other toys before they started producing airsoft guns. The recession in the 90's caused the company to just go back to producing airsoft guns instead of R/C cars and other toys. Since then the company has stuck with producing airsoft guns and has also produced a series of model trains. Such is the popularity of its guns that the company has its own center for airsoft called Tokyo Marui BB Sports Field. As its products are notable for their detail and accuracy, they have appeared in numerous movies, such as The Matrix. Tokyo Marui is widely known as Japan's largest and most renowned airsoft gun manufacturer.
The vast majority of the airsoft guns used in the show are made by Tokyo Marui, and Tokyo Marui's logo can be seen in the credits after each episode because they sponsored the show. Tokyo Marui dedicated a page on their website to C³-bu and it shows a list of their airsoft guns used by the main characters, however their list is not entirely accurate. All guns in the show are assumed to be made by Marui, however some cannot be TM-made, either because they don't make a replica of that model, or their replica differs from the one seen in the show. The only guns and accessories not made by Marui (used by named characters) are as follows:
Sonora Kashima's M60, presumably made by A&K. The A&K is far more likely than the TOP M60, which is an old, now discontinued replica and had reliability issues.
Yachiyo Hinata's P228, made by WE. Marui's P228 is spring powered as well has having minor cosmetic differences to Yachiyo's/the WE. It's the WE over the Tanaka due to the WE's distinctive finish.
The claw-mount red dot sights on Rain Of Bullets' MP5K-PDWs, presumably made by G&P. Out of the replicas of this sight, the G&P is both the highest quality and more importantly the lens has the same orange tint the one in the show has.
The Raptor-II suppressors on the same MP5K-PDWs, made by Madbull.