I've never seen any manufacturer RECOMMEND a particular ammo. I've seen some say NOT to use certain ammo; but never endorsing a particular ammo.

As for if a weapon is mil-spec or not, I think is a totally different subject. This is a "s&W" specific forum. As such, that is what is generally implied. But I consider Mil-spec, when it comes to civilian rifles like the AR, to be quite a bit overrated. And that's even after spending 21 years in the military dealing with Mil-Spec. Not one of my hunting rifles are "Mil-Spec" compared to a 1903 springfield or similar. Yet; my hunting rifles are a lot more accurate than any AR. Do I care if my barrel is 1:7 twist and 4150 steel instead of 1:9 and 4140? No. I don't shoot 70+ grain bullets and my 1:9 is chromed; so it isn't important to me. Same with the BCG. Do i really care if it's an M16 FA bcg instead of an AR15? No. I don't shoot full auto and the AR15 bcg has proven to be quite reliable. If by chance the chamber was modified to accurately fit steel case ammo; would that bother me (Because it's not mil-spec)? Not at all, as long as the rifle was still reliable, dependable, and accurate.

The point is; I bought an M&P15OR for a number of reasons. Quality manufacturer; quality parts; reliable; not plastic; eats any ammo you feed it; iron/red-dot open sights get me an acceptable group; nice trigger pull; etc... Do i care if it's 100% mil-spec. nope; not in the least. Except for 2 other true military rifles, the rest of my weapons are not mil-spec and they function perfectly.

No people are free to not shoot steel case ammo if they want. They are free to shoot it. I shoot around 400 rounds of .223 a month. I'd rather spend $80 doing that at $4 a box than $200 for $10 a box. Every rifle is unique. If a person tries steel case and it works good in their rifle, then they have a very good financial option here to shoot steel case. It will save them a lot of money and it will work fine.

FWIW: I have a 20 year old frankenstein AR that was built before steel case ammo became popular. I know for a fact that the various manufacturers who built the individual components had no idea of building their parts with steel case ammo in mind. And that frankenstein will eat steel case ammo perfectly every time. And when I'm done shooting my 300-400 rounds in an afternoon, I clean the gun (10 minutes of time), and it ready for next time. And there is no premature wearing or defects in the weapon because of steel case ammo.