Last time I sent one back it was via FedEx. S&W sent me the label by e-mail, I printed the label, attached it to the box, and FedEX came by the house and picked it up at a pre-appointed time. Make sure you remove any aftermarket items (grips ?) on the gun and the magazines.....and don't send the original box/case. If you send these items, you may never see them again.

How long it will take depends on the work being done, and how far behind they are at the moment.

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Ret. LE, FA Instr, S&W Armorer