Id just contact the customer service line or service rep of S&W. They have
lifetime service plans. If anything, you could just get a new SIGMA .40 to shoot.
I wouldn't waste any time with gunsmiths or the FFL holder where you purchased it.
Many of the local FFL holders & gun shops near me have a "no refunds" or "no return of firearms" policy. If you'd ask for them to help you get a replacement or repair, they'd act like you're from Mars.