www.aimsurplus.comI've personally ordered thousands of dollars worth of guns, ammo, and equipment through AIM. They are my #1 go to for gun related items. They ship promptly (and I think only charge cost for UPS Ground), have great customer service, and great prices. During the ammo rush, they didn't jack up their prices like other dealers (of course, that meant that they were sold out a lot, but it was good to see a company NOT trying to gouge people like a certain site starting with C).
AIM is a wholesaler, so their inventory fluctuates, it's best to often check their site because when they get a good deal in, often times retailers buy up all their stock of guns or ammo. They are also a great place to find magpul and daniel defense products, I think they have some deal with them, they seem to have magpul stuff in stock when everyone else is sold out.