Difference between 223 and 5.56

[Fire/hunting guy]

I'm looking into building me an AR and I'm kind of a noobie when it comes to them.  What kind of lower should I be looking for?  Also, I think the 5.56 will fire the 223 round, but not vice versa, is that right?  Any help would be greatly appreciated.

[Fire/hunting guy]

Dang it, didn't scroll down to long guns apparently, maybe I should go get my glasses!

[TripleDigitRide]

A 5.56 can also handle .223. The opposite is debatable, or so I've seen it debated. 

 

As far as the lower goes, there are tons of AR15 lowers to choose from. You have polymer, forged, billet... 

 

I always try to buy the best my budget will allow.

[Dolomite_supafly]

Dimensionally loaded 223 and 5.56 is identical. The difference is pressures. SAAMI spec for 223 is 55K psi and 5.56 is more than that. Most estimates is 5.56 runs 65K.

Now because of the additional pressures of 5.56 the chamber is slightly different than a 223 chamber. The chief difference is the throat in a 5.56 is slightly longer than 223. This allows the bullet to begin moving, and reduce pressures, before hitting the resistance of the rifling.

Because of the longer throat of the 5.56 chamber it is safe to shoot 223 in a 5.56 chamber.

Some say that you cannot shoot 5.56 in a 223 chamber. But I have shot a lot of 5.56 out of guns with a verified 223 chamber. Shooting 5.56 in a 223 will not cause some sort of dramatic failure. If it does then your gun had issues in the first place. Proof loads are well beyond the pressures encountered when shooting max 5.56 loads in a 223 chamber.

Then there is the estimate that 25% of the "5.56" actually have 223 chambers. As manufacturers chamber barrels the reamers dull and most resharpen the reamer. This reduces the size of the reamer each time. There was a random test done by a publication and well over 25% had 223 chambers so if shooting 5.56 in a 223 chamber caused the gun to blow up we would be seeing it everyday.

[Smith]

What Dolomite said. The only issues I've heard that seem like they could actually happen are that the higher pressure over time can cause throat erosion and possible gas port erosion. It seems like that is possible, but also seems like it would take A LOT of rounds before it could be an issue.

[Guest G Carr]

Hi everyone ... First post. I understand and totally agree with Dolomite and Mr Smith. Its all about manufacturing tolerances and quality control. Question is why even consider a barrel chambered 223 ???

[CM1021]

+1 for what dolomite said. It also depends in if you want this gun to be a precision gun or just a "plinker." I say plinker that way because some 5.56 chambers can be pretty accurate. But if your wanting a sub MOA gun go with a .223 or to throw something else in the mix a .223 wylde chamber. Fwiw all my guns are either 5.56 or .223 wylde. I have an 18" SPR clone and it will shoot .5"-.75" with my handloads so it can be done with a "looser" chamber.

[peejman]

As usual, dolomite is right on. 

 

That said, the lower has nothing to do with it.  The only part that matters is in the barrel. 

[Dolomite_supafly]

I actually prefer a 223 chamber. Reason is it has more going for it as far as accuracy goes than 5.56. Not saying every 223 chamber will outshoot every 5.56 chamber but the 223 chamber starts out with tighter tolerances which helps accuracy.

 

Also a 223 chamber doesn't swell brass like 5.56 can and that gives you longer case life.