300 ammo is much cheaper, and you get a better bullet selection and heavier bullet options than the 6.8. Plus 6.8 ammo is very hard to find these days.

Theortically, yes. But by the time you switch just the barrel, and remount the scope, you normally get a shift in POI from different bullets and calibers. I want a different scope anyway from my 223 and 300 black out, so I need seperate uppers with each having their own scope.

I, as well as MANY, don't need a choice of ammo that differs by 100grs. Also, I have absolutely no problem finding 68 ammo any day of the week. I don't understand where that is coming from. I can buy 6 or more different brands/weights/types of 68 ammo right this second.

Here's 14 different choices from only ONE manuf.!
http://www.ssarmory.com/6.8ammunitionsales.aspx

Hard to find 68 is a thing of the past!!!!!!!

There is no doubt in my mind the 5.56 makes the most sense for a training, 3 gun, varmint or a HD round, they feed with no issues, everyone makes mags and every part is cheap. There is just no need to look for any other caliber when considering the uses above.
The 6.8 has a dual use as well, it can be a very effective legal hunting cartridge and a HD round also. The terminal performance on game is much better than most other cartridges, making it the best choice for deer and hogs. Several Goverenment agencies have moved to the 6.8 because it offers 80% of the power of a 308 on a much lighter platform. It is the second most popular AR15 cartridge in terms of sales and suppliers.

I can walk into Academy right now and buy 6.8. This hard to find talk is nonsense.