Ryuuken's weapon is the standard Quincy bow and reikyuu (spirit arrows). Quincy create their weapons by drawing spiritrons out of the air and combining them with their own spiritual energy. The larger the bow and arrows, the more spiritual energy is being expended, seems to be how it works.

Initially, Ryuuken used a small bow that he operated with just one hand. It was enough, however, to take out one particularly large Hollow. He used a bow of the same size to chase Uryuu around a bit when they were working on getting his spiritual powers back.

The bow changes shape after Uryuu counterattacks. The reikyuu is strong enough to completely dissolve Uryuu’s Grits attack. Although, Ryuuken points out, if Uryuu had any brains he would have used Haizen instead of Grits. Of course.