Is the gun mention a subtitling error?
I think the subtitles are okay. The Naruto manga does use a similar gun analogy. In chapter 70:


  ...Like a gun barrel when the path of a bullet is blocked, Zaku blew his arm off.


In the anime, I believe that the word that Dosu is using is 砲身 (ほうしん), which translates to "gun barrel". For context, in that sentence, Dosu was saying something along the lines of:


  砲身【ほうしん】であるザクの腕【うで】は暴発【ぼうはつ】し使【つか】い物【もの】になるなくなる