清朝时已经懂得用步枪了，为何与西洋作战时却不使用，而仍使用传统的冷兵器？
因为你觉得“与西洋作战时却不使用”。
清军在18、19世纪本来就是以步枪为主要装备。
1900S的清军
1850S的清军
以下节选一段描述天津天津保卫战的资料：《The savage wars of peace》 Max A. Boot。 ISBN 0-465-00721-X
The main effort against the South Gate became pinned down in an exposed position under Chinese fire from within the city. The Chinese again unleashed a torrent of fire upon the Allies. The allied troops were forced to lie face down in mud. The uniform of the American troops, dark blue, provided a virtual bulls eye to the Chinese troops, equipped with western firearms, such as Winchesters, Mausers, Mannlichers. Herbert Hoover led American Marines to assist the Royal Welsh Fusiliers, and they hit a swamp where Chinese guns murderously targeted them. The Chinese kept the guns firing for over five hours, until nightfall. Bullets were described as "hail", and guns provided a "death tattoo".
大致翻译：进攻中由于缺乏总体指挥，对八国联军的不利影响很大。通讯不畅和行动不协调困扰着攻击方。在城内中国炮火的攻击下，进攻（天津）南门的主力被压制在开阔地上。清军再次向联军开火。联军被迫面朝下卧倒在泥地里。美军的深蓝色制服，为中国军队提供了一个良好的靶子，他们配备了西式武器，如温彻斯特，毛瑟，曼利夏。海尔伯特胡佛率领美国海军陆战队协助威尔士皇家燧发枪团，他们穿过一片沼泽地时，中国的枪炮凶狠地瞄准了他们。中国军持续开火五个多小时，直到天黑。子弹像冰雹一般，枪响像是鼓点。
（后面是联军以火力掩护 爆破组反复对清军阵地实施爆破，最后是日军以自杀式攻击炸开通路，联军获胜）
