my 3913 has a lug (ring) completely around the barrel which fits into the slot milled into the slide. Your barrel (fuzzy picture) appears to have this lug missing but there is some sort of ring looking line. Your slide has a lug groove just forward of the slide ejection port. Maybe S&W decided that the barrel lug was no longer a design necessity. ALL my other S&W autos have the barrel lug. something does not seem right. I would contact their customer service. pete