In various embodiments, a surgical stapling instrument can include a handle, a shaft extending from the handle, wherein the shaft defines an axis, and a disposable loading unit which is assembled to the shaft in a direction which is transverse to the shaft axis. Such a connection between the disposable loading unit and the shaft can prevent, or at least inhibit, the disposable loading unit from being unintentionally displaced proximally and/or distally relative to the shaft of the surgical instrument. The surgical stapling instrument and/or disposable loading unit can further include a threaded collar and/or detent assembly configured to hold the disposable loading unit in place. In various embodiments, a disposable loading unit can include a lockout feature which can prevent, or at least inhibit, an expended disposable loading unit from being reassembled to the elongated body of the surgical instrument.