Well, in short yeah you can. Are there issues with it, well maybe so..... Will it blow your gun aparts and send pieces of the gun thru your brain housine group, probably not. But lets take a look at what the issues are for reals.

First of all there are three major types of AR chambers. The .223 SAMMI, the .223 Wylde and the 5.56 NATO. It's all so crazy and convolutted that we will skip right to the point of it all and look at it in laymans terms.

The real issues are the case neck, freebore length, throat angle and headspace. Now what do they each mean?

Case neck- Thats where the bullet sits in the case.

Freebore length- Thats the length between cases end and where the lands and groove start. This is where the bullet is actually free in the bore when it leaves the case. This affects the pressure of the round as the lands and grooves cause pressure to build behind the bullet.

Throat angle- This is the angle of slope where the freebore ends and transitions into the lands and grooves.

Headspace- This is the depth of the chamber

Now the headspace on all chambers is the same at 1.4636, so that is a non issue. The throat angle is significantly different though, 3 degrees for the SAMMI and 1.25 degrees for the other two. How does this affect you? It makes the pressure in the SAMMI start quicker as pressure is appled more abruptly.

Freebore length is a big issue in the three. The freebore for the SAMMI is a mear .025, the Wylde is .062 and the 5.56 NATO is .057. What does that mean? It means that the SAMMI builds pressure a lot faster than the Wylde or NATO chambers. Add this to the fact that NATO rounds are already a few hundred feet per second faster and as much as 5,000 PSI higher in oressure and you have an issue building rapidly. As is the pressure.

Does this mean that your gun is going to explode by using NATO ammo in your .223, probably not. They are massively over engineered and generally pretty safe for these kinds of things. They are designed to take pressure spikes, however I DO NOT RECOMMEND IT. It simply is not safe. You WILL probably start to see primers popped out of the cases or swollen.

So what do we do about it all? Well, we try to order all of our AR's in the 5.56 chambers. That allows you to shoot whatever you want to in the gun. The next thing you can do is shoot .223 ammo in your guns. Milsurp is not as cheap as it used to be, so there is not much advantage to go that route anymore. But if you do find a deal having a 5.56 chamber is going to make life more comfortable for you. But if you dont have a 5.56 chamber, just dont shoot milsurp ammo.

It's not exactly the highest-skilled job in the gunshop to ream a .223 chamber to 5.56.  Probably take all of five minutes, if you take time out to pop a top and light a cigarette.  Well, okay, maybe ten...

'Rat

My question is it safe to fire 5.56 through a Mini-14? I picked up a can of surplus 5.56, and from what I've read on firing 5.56 out of a Mini, it isn't an issue, but I'd like to get more opinions on this.

So, if an AR has .223/5.56 stamped on the side, is it good to go with either?

Thumper, exactly.  You're GTG.

Cool!   Thanks, bro!

Also the Kel-tec SU-16 series is (according to the nice tech dude I spoke to there) chambered 5.56 NATO, and is, therefore, good to go with .223 ammo.

To piggyback to this thread.  Anyone know if you can shoot a steady diet of 5.56x45mm in the new Ruger Mini 14s?  I wrote Ruger but have not heard back.  I aksed on on another thread and no one seemed to be able to answer with a definitive yes.

Thanks.

To piggyback to this thread.  Anyone know if you can shoot a steady diet of 5.56x45mm in the new Ruger Mini 14s?  I wrote Ruger but have not heard back.  I aksed on on another thread and no one seemed to be able to answer with a definitive yes.

No, the Ruger Mini 14 was designed specifically to shoot .223.  While it might work, it was not intended to handle the pressures found in 5.56NATO rounds.

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Harry is correct in many LEO and Dept. of Corrections that use the
Mini 14s, use the GI ammo    My 181 series digests both without
a difference in function or accuracy.   Handloads have tweaked the
accuracy somewhat, but a target rifle, mine is not.   Yep, a keeper
for fail-safe and reasonable short range rifle     Maybe a bit over
priced, but what isn't?   Price of high cap mags is sinful    How-
ever mine even uses the the cheaper aftermarket without a hitch.
I do load a few rounds less than capacity.   If you wonder about the
223/Military, think about this.  Pushing a 311/312 bullet down a .308,
as in the Mini 30.  It's in the chamber and probably some freebore.
Don't own one of these, but I'm told these are not MOA either, but
still work great!
   Nope, don't think I will rechamber my chrome barreled Bushy, and
don't hesitate to use either.   Also handloads in the same pressure range
are not a concern of mine.   For the type chamber recommended that I
understand will improve accuracy for some, I would order an upper with
those specs.   The chrome moly would work for a rechamber, but make
sure you use a recommended  smith to do the work, or THE FACTORY!

+ 1 on the barrel markings. There are lots of home built AR's out there.

Always go by the barrel marking. .223 ok in 5.56 chamber 5.56 in a .223 chamber NOT OK! As far as Mini14 goes and this is from a Ruger rep I know, The new Mini's are ok for 5.56 the older ones are not. I don't have the date that this changed but at some point they either started using 5.56 chambers or strengthen .223 chambers.