AR Upper Receivers:  I saved the best for last, this is where most of the MCG action has been in the last 10+ years.  An entire new family of cartridges has been created around the constraint of the AR-15 magazine well width and AR-10 cartridges like the .243 Winchester have gained popularity as well.  Buying an upper is almost always going to be less expensive than a complete rifle, and if you put a lot of money into a lower with an aftermarket trigger, high-end stock, and grip why not stretch that out to several platforms?  Of course, the big drawback is

 In the last 10 years or so this is where all of the advancements for MCG have been made.  An entire new family of cartridges has been created around the constraint of the AR-15 magazine well width and AR-10 cartridges like the .243 Winchester have gained popularity as well.  If you're looking for something less expensive then purchasing an upper over a complete rifle is almost always going to be the best option, and if you put a lot of money into a lower with an aftermarket trigger, high-end stock, and grip why not stretch that out to several platforms?

Of course, the big drawback is one lower, one shooter — bad if you need to defend your retreat and none of your buddies bring a rifle.  Some may come to the conclusion that 2-3 complete ARs are better than one lower and 5 uppers.  If you’re going to make the leap, I am of the opinion that a 5.56x45mm base rifle + pistol caliber matching your sidearm + 6.5 Grendel long barrel with scope + .22 LR conversion kit would be the most effective and efficient setup.  Note that, even pinching pennies with lower end upper assemblies, this will be almost $3,000 before optics.  For $2,500 you could buy a basic AR, an inexpensive pistol carbine like a Hi-Point or Kel-Tec SUB2000, a budget long range .308 bolt action rifle, and a .22 LR kit for your AR (or basic Ruger 10/22 rifle) and have 3-4 complete guns.  It’s not for everyone and your mileage may vary.  I honestly don’t see much utility in multiple uppers for AR-10s as, beyond .308 and .243, the cartridges are just not all that common.  SHTF firearms that’s highly portable versus a huge, difficult to move stockpile at your permanent live-in retreat.