I mentioned how DSG gave me a "significant" discount on the Cav Arms lower receiver. To get their military discount, I sent them a copy of my mil ID, and they "upgraded" my account so now whenever I log into their site, it shows the normal price, the discounted price, and the savings. The discount isn't a flat rate, but it's different on every item. Not every item is discounted, but the majority are.
Then I placed an order with Brownell's, and as I was checking out it asked if I wanted to register for an account. I figured why not, since I am building a rifle from all parts, I know I'll be buying more from them. There was a little check-box at the end asking if I was active duty military or law enforcement. I checked "yes." Lo and behold, they do the same thing as DSG. Again, not every item is discounted, and the discounts are all different, but some are significant. I saved about 15% on my last order. Good enough.
So far, I haven't found a discount on Midway, but I'll still keep checking their prices.
Of course, there are tons of other sites and companies that sell gun parts out there, and I shop as many as I can for the product I want for the best price. but Brownell's and DSG are getting a lot of my business because of their discount programs.