With ANY AR manufacturer, wait time will depend entirely on the configuration you order. Bushmaster has over 40 barrel configurations alone, and they sell them out faster than they can make more, and so they are often out of stock on the less-frequently-ordered barrels. But sometimes you get lucky, and that barrel will be a configuration that they just made a run of, and they'll ship the next day.
Also, sometimes other parts, like bolts or carriers, or uppers will be back-ordered (military and LEO orders are sucking up the supply of these from the top-tier manufacturers). These common parts aren't usually out of stock very long, though.
Finally, and again this is true of ALL AR manufacturers, the priority is as follows:
1. Military orders
2. LEO or government agency orders
3. Complete rifles
4. Complete uppers/lowers
This may not seem fair if your waiting on individual parts, but wouldn't you run YOUR business this way? I would.