I have 2 lower receivers from the same company. One is .223 and the other is 5.56.

If I build the rifles up, one designed to shoot .223 and the other to shoot 5.56, what are the differences
between the two in parts?

I read that I can use .223 in a 5.56 rifle, but not the other way around.....or is that backwards???

DUPE!!!    j/k

I believe the chambering is the only difference.  And yes, you can shoot .223 in 5.56, but NOT the other way around.

ETA
You can, just don't do it.

Most Good AR-15 barrels are chambered for 5.56 so they can use both. However most bolt guns aren't chambered in 5.56, but .223. So using 5.56 will result in stuck bolts. The difference between the two is that 5.56MM is a max NATO loading with much higher pressures.

The .223 and 5.56 rollmarks on the lower mean nothing. You can build them both up as 5.56 rifles if you choose. Both lowers are the same.

There's NO difference.

There is no difference in a lower receiver marked .223 and a lower receiver marked 5.56mm.

There may be an important functional difference in a .223 AR barrel and a 5.56mm AR barrel though. You can always use .223 in a 5.56 chamber; you can't always use 5.56 in a .223 chamber.

So then, if I am to build them up, what are the difference in parts? Chamber and barrel and everything else is the same on the 2 rifles? Or is it better to build both as 5.56, this way I can shooet .223 in them? Will I lose accuracy?

I also read the .223 is more accurate than 5.56, but not as powerful.

Quoted: So then, if I am to build them up, what are the difference in parts? Chamber and barrel and everything else is the same on the 2 rifles? Or is it better to build both as 5.56, this way I can shooet .223 in them? Will I lose accuracy? I also read the .223 is more accurate than 5.56, but not as powerful.

5.56mm isn't more powerful really. It is just loaded a little hotter and travels a little faster (50-100fps) than commerically loaded .223. Because this bullet is very dependent on velocity for its wounding effects, the 5.56mm gives a bit more range.

A .223 chamber has a shorter throat. This helps accuracy but reduces the amount of crud that can be in the chamber before the weapon will not work. A 5.56mm chamber has a longer throat and more powder. If you fire 5.56mm in a .223 chamber, the combination of less space (shorter throat) and more powder make for very high pressures that affect function.

Personally, I would go with a 5.56mm chamber. It is already more accurate than most of the people who will shoot an AR15, allows flexibility in ammo, and is more reliable. However, if you think you are going to be a sub-MOA shooter and really want the best you can get out of accuracy, consider a Wylde chamber - it is a compromise between .223 and 5.56 and will fire both types of ammo with very good accuracy.

As far as difference in parts, how the chamber is cut is the only important difference here. If you put a barrel with a .223 chamber on a 5.56mm marked lower, you still have a .223 rifle.

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