IMHO........At this point, "Don't worry so much about the theory."
Go shoot.
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Consider this...............223 Rem. And 5.56mm chamber specs
http://www.radomski.us/njhp/cart_tech.htm
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For those considering a new barrel............
Pick, what YOU figure, will be best FOR YOU. Simple.
Twists, chambers, makers, profile, weights, etc..... They vary.
The mag length "restriction is always there."
Different bullets mfns offer bullets in: various ogive's, weights, construction, measured lengths, etc....
Single loading is always a possibility when using "longer" bullets. And, there are magazine modifications available that might get you loading single cartridges with more confidence.
You could always have it "custom made."
But, in the end.........you've picked what you wanted. Then, you go shooting.
Aloha, Mark