You cannot be in possession of both an AR receiver AND an AR barrel of less than 16" length. If you possess an AR receiver and buy an SBR barrel wanting to permanently add length to reach 16", you'd be in violation and vulnerable for the whole time it takes to do this. Either buy the barrel already modded or get the receiver out of your possession while you have the barrel in SBR length.
The violation here, BTW, is "constructive possession," it means you have an illegal weapon by just having all the individual uninstalled parts under your control.