I cannot stand to use FEDEX for anything unless it is absolutely necessary. I think they are clowns pretty much throughout the entire state of Texas. They've become very unprofessional/unreliable and arrogant with regards to customer service. I haven't been able to trust them for years. My list of complaints with them is endless. The USPS here has a game of pursuing their own agenda. In that past couple of years, every time I order a gun part and it is shipped via the USPS it seems the package has been opened or the outside colored plastic has been scratched so somebody can see through the bubble wrap and figure out what's inside. I just received a package from MidwayUSA that way today. They also have a habit of using "discretion" and will on their own require a signature for a package when they seem to suspect that gun parts are being shipped. They have done this even when I've ordered something and specifically requested it to be shipped without requiring a signature. I've had good luck with UPS here lately.
I'm glad you posted the information about the DPMS high vs. low rail. I never even heard of this and I am trying to figure out what my final configuration will be. I want to use the traditional A2 front sight post with a Magpul hand guard but I don't know if this is possible, nor I currently have no clue what barrel nut I will need for that specific configuration.
Hope your barrel gets to you soon.