Abstract: A barrel comprises a cylinder wall, two parting lines, and an external thread. The external thread is intersected by the two parting lines respectively, and the external thread includes a plurality of threads. The plurality of threads are connected spirally and each of the threads includes a pair of normal thread portion and a pair of shrinking thread portion. Each of the normal thread portions includes a normal thread crest and a normal thread root. The plurality of shrinking thread portions are intersected by the parting lines respectively, and each of the shrinking thread portions includes a middle location intersected by one of the parting lines, a shrinking thread crest, and a shrinking thread root. The outer diameter of each of the shrinking thread crests reaches a minimum at the middle location, and the minimal width of the shrinking thread crest is smaller than the normal crest outer diameter. The width of each of the shrinking thread crests is larger than the normal crest width.

Abstract: A barrel comprises a cylinder wall, two parting lines, and an external thread. The external thread is intersected by the two parting lines respectively, and the external thread includes a plurality of threads. The plurality of threads are connected spirally and each of the threads includes a pair of normal thread portion and a pair of shrinking thread portion. Each of the normal thread portions includes a normal thread crest and a normal thread root. The plurality of shrinking thread portions are intersected by the parting lines respectively, and each of the shrinking thread portions includes a middle location intersected by one of the parting lines, a shrinking thread crest, and a shrinking thread root. The outer diameter of each of the shrinking thread crests reaches a minimum at the middle location, and the minimal width of the shrinking thread crest is smaller than the normal crest outer diameter. The width of each of the shrinking thread crests is larger than the normal crest width.