Just spotted a goof!
During the famous walk scene, Pike Bishop's Winchester 1897 shotgun has barrel that is equal in length to the lower magazine tube. Minutes later during the climactic gunfight, his shotgun barrel extends several inches past the magazine tube.
I just submitted this to the goofs section.
Walk shotgun:
http://picturearchive.gunauction.com/2017949/11632429/wildbunch.jpg_thumbnail0.jpg
Bloody Porch shotgun:
http://picturearchive.gunauction.com/2017949/10578143/wild-bunch-movie-02.jpg_thumbnail0.jpg