If you got that off of the retailer's site, it almost certainly displays that SIG is issuing a completely new SKU selection (outdated SKU is discontinued) to support a change. That new structure usually means the MCX bolt can operate without the addition of a buffer tube/inventory too. (Photo: Jeff https://sig-sauer-firearms-for-sa88642.shotblogs.com/sig-sauer-mpx-k-for-sale-fundamentals-explained-35682692