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. As far as the best grain weight. The most common two are 55gr & 62gr.

Hi Albert and welcome. Yes, a 5.56 stamped firearm can fire .223 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!

Yes, .223 is fine. "Best" depends on your intended application. 55 grain is certainly the most common, followed by 62. If you are just target shooting/practicing, I would say pick one that you can get a large quantity of for a decent price and then consistently use that particular weight. Other lighter or heavier options are generally lighter weight varmint type rounds, heavier weight precision/long range for purposes, or different hunting or defensive rounds design for expansion.