I was jesting - to a degree- about the "MBR" - i.e. where is it ?
I like arnoldrew's comment - "my fightin' gun". IMO, there's nothing essentially wrong with the 5.56 M4; it's perfectly capable as is. Nope, I don't drink the koolaid that says you need .308. Most don't; 5.56 will do the trick for most things.
However, at the range, I like to shoot water bottles, clays, stuffed animals, etc. A .22LR is only so much fun with these. 5.56 - a bit more. .308 tends to be quite the party animal. .300 Win Mag- just plain awesome- but pricey.
Congrats on your big 5 though. It's a good mark.. Really like the pistol... I imagine that 7mm is a thrill to shoot, but pricey. I know- have a .300 Win Mag that does not see enough range time. FWIW- I bought the .300 WM with credit card points- no cash out of my pocket. Semi-auto to boot.
My initial thought was to use the .300 WM for the fun stuff; sounds like a cannon. But pricey- it's significantly cheaper with .308 in bulk- especially for range stuff.
I would suggest a .308 - just because it's cheaper in bulk- and .308 is more fun for blowing stuff up

. Range ammo for .308 in bulk- is about 70¢, compared to 5.56 at 50¢.
I don't know other people's opinion on reloading; everyone I've heard suggest that it's usually for accuracy more than costs saving. The time,effort, & risk does not make it worthwhile- yet- If I had a range outside my back door- I would though. To try out a few reloaded rounds- for me- takes the better part of an afternoon. I do collect my brass- just in case ...