Well, principal difference in 5.56 Nato and 223 is pressure loading, followed by overall length. The overall length is what causes problems. The bullet being jammed into the lands in a short throated chamber is going to build significant pressures. Most factory rifles are oversized (IMO) chambers so they will chamber any brand of ammo. Savage does a decent job of keeping the chamber smaller, but any mass produced firearm is going to have production run variances, as tooling wears.... An early run will have a larger chamber than a later run as the reamer wears smaller cutting multiple chambers. Irregardless, they usually also have long throats (where the lands meet the bullet in the barrel.... a short, smooth area slightly in front of the chamber) hence the reason handloaded ammo is usually more accurate than factory, as handloaders often seat bullets much further out so the bullet meets the lands, giving the bullet less wobble when starting down the bore..... The Axis I have has a pretty long throat, allowing the bullet a significant jump before starting down the bore. This reduces pressure quite a bit, so the 5.56 ammo is not showing any signs of pressure whatsoever. Same story for any of the bolt guns I have in .223.
FWIW, Savage is probably the only one to offer ANY rifle in a twist faster than 1/9, offering the Lang Range Precision Varminter in a twist as fast as 1/7. They also offer rifles in 6.5/284, 260, and 6.5 Creedmoor, IIRC.

Great starter rifles, asides from a crappy trigger group, and probably the best "bang" for your buck. Granted, they aren't the Axis line, but speaking with an engineer late last year, the 260 may soon be part of the lineup, as well as the 7mm-08, in the Axis line.
Lewis Machine and Tool are excellent rifles, custom made, but sooooo overpriced, IMO. They are not tolerant of much dirt and play time, from my experience.
Barrels are a beast in themselves, between Stainless Steel, Chromoly, chrome vanadium, chrome lined, and carbuerized (nitrogen purged, such as Glock's Tennifer process) there are so many selections available it'll make your head ache! For everday shooting, SS, chromoly, and vanadium are all excellent choices, while in full auto rifles, chrome lined and carbuerized barrels are a necessity. Then there's the size of gas hole, gas block orientation, tube length, piston or gas driven bolt, and ammo selection. Surplus USMC barrels are a good example... basic assembly works excellent with surplus 5.56 nato ammo, but a lot of commercial ammo won't correctly cycle the bolt. Enlarging the gas hole in the barrel or reducing the buffer spring tension is necessary to facilitate correct function.