I'm new to this and don't know the exact reason but someone correct me if I'm wrong. The factoids I've found:

1.) A barrel stamped 5.56 can shoot 5.56 & .223.

2.) A barrel stamped .223 should only shoot .223.

3.) Leade = the distance the projectile has to travel before it contacts the barrel's rifling.

4.) .223 leade is shorter than 5.56.

5.) .223 & 5.56 are practically the same dimensionally. The difference is in the powder charge. Powder Charge 5.56 > Powder Charge .223.

6.) Since a .223 round is designed for a shorter lead, it will be more accurate in a .223 barrel because the projectile contacts the rifling sooner. The "tighter" chamber to leade does not not require as much powder charge in a round of .223 v.s. a 5.56.

7.) A 5.56 round is designed for a longer leade. The "looser" chamber to leade allows for the higher pressures the increased powder charge generates in a 5.56.

I'm really waiting for a more experienced member to chime in. I've read posts where some individuals swear it's ok to shoot 5.56 out of a .223 barrel. Some say it's not a good idea.

As for me, I chose my M&P 15-Sport because it has a 5.56 barrel. That way I know I can safely shoot 5.56 & .223.