For those of you who are actively building, have built, as well as for the knowledge benefit of those who are considering builds in the near future, let's discuss what parts and portions that are the hardest to get at the moment.
1) BCGs- A good M16 BCG at a fair price (under $200) is nearly impossible at this moment. You'd better have an inside track with a really good source. 6 months ago you could have all the BCMs you wanted for $170 delivered. Now they're $225+, if you order within 17 seconds of receiving your email alert. Any longer and they're sold out. The fact is, the BCG supply had run a bit dry even before recent events. Add in a healthy dose of hysteria, and they're almost worth their weight in gold. I sold a couple used DPMS AR versions on Ebay for about $285 a piece in January. That's not the price I asked, that's how high the bidders chose to go. They were probably worth no more than $100 a piece. Without a little Jedi magic, it's tough to do much better than indefinitely backordering one at a respectable price. Maybe you get it in 11 weeks, maybe 11 months.
2) Stripped Lowers, Barreled Uppers, Mags, Ammo- All are tough to find at rational prices. But they're all out there if you're on the right backorder/email alert lists or possess a steady network. Some folks continue to pay 200-300% markup prices on all of these.
3) LBK/LPK- Critical in completing your lower. I'm a big fan of the PSA/Magpul LBKs. They've been up for sale multiple times in the last several weeks for very fair prices, and allow you to finish a quality lower build with one modest purchase.
4) Charging handles- A little tough, if you want a specific model from a specific company that you're already placing an order with. But they're easy enough if you look around.
5) Everything else (sights, slings, optics, accessories)- Easy.