This is true, due to 5.56x45 developed for the military, and .223 Rem was developed for civilian use. Military 5.56 has a larger chamber to accept dirty ammunition so the shoulder of a 5.56 has more play if that makes sense. 5.56 is also a hotter round.

Hi Albert, yes sir, you were told correctly. You can fire both 5.56 and .223 in your Diamondback.
If the barrel is stamped .223, then you can only fire .223.
Stay safe & have a great day!

Absolutely 💯 correct.

It is true.

Good morning to you!! A barrel marked 5.56 NATO will accept both .223 and 5.56. But a barrel marked 223 will not accept 5.56 NATO. 55 grain ammo is a good all around bullet weight. Hope this helps. Have a good rest of your day.
🇺🇸🇺🇸 'Merica' 🇺🇸🇺🇸

Yes, what you were told is correct: you can safely shoot .223 Remington ammo in a firearm chambered for 5.56 NATO, but you should not shoot 5.56 NATO ammo in a firearm chambered only for .223 Remington because the 5.56 NATO round is loaded to higher pressures, potentially causing dangerous overpressure in a .223 chamber,

Hi Albert and welcome. Yes, a 5.56 "higher pressure round" stamped firearm can fire .223 "lower pressure round" ammunition no issue. Bullet grain will depend entirely on the type of shooting you are doing. A rule of thumb is to use a similar grain weight bullet for practice that closely matches your self-defense ammunition. This makes the point of impact similar. 55gr and 62gr are used most in an AR. Cheers, stay safe and enjoy!

That is technically true, yes. And by following it as a general rule, you should never really experience any problems unless using very low quality or questionably hand-loaded ammunition. The concern of chamber pressures and safety is likely overblown for today's firearms though, it quite likely barely matters if at all.