A couple of points:

First, ALL AR manufacturers prioritize their parts as follows:

1. Complete rifles
2. Complete uppers
3. Parts

So, if you order just a barrel, just an upper receiver, just a bolt, etc., you are 3rd priority IF there are not enough parts in stock for everyone.  That's just the way it is, and it applies to everyone.

Second point: you were ordering a barrel configuration that doesn't sell in high numbers, which means that runs of that configuration are only made once in a while.  You happened to order when none were in stock, and had to wait until enough orders accumulated to justify another production run.  Had you ordered a more common barrel, such as a POST-ban 16" HBAR, you'd probably have gotten it in a few days.  Bushmaster makes over 40 barrel configurations, and they can't stock them all in sufficient quantity to ensure that there will be one when YOU call.  Remember, they ship more rifles, uppers, and barrels for civilian/LEO sales than anyone else in the industry, by far.  It isn't like they aren't trying.

Third point: you are *assuming* that Davidsons put in their order after you, and received their rifles before you received your barrel.  And, you *could* be right (see 1st point), but I doubt it.  Davidsons orders ARs on a regular schedule, and they likely submit those orders FAR in advance to ensure they receive them on time.

-Troy