I have noticed at academy Federal 223 REM box shows a muz velocity of 3240 FPS  and Federal XM193 5.56 loading the box shows muz velocity of 3165 FPS.   That does not compute for me. Why would a 223 loading be significantly higher velocity than M193 ? Both loadings are 55 grain FMJ.

Anyone else notice this?  Shouldn't the .223 REM be lower velocity saami spec?

Did they use the same barrel lengths during their testing?

I'm just going by the box.  I just checked the federal website and it shows the same specs.  Oddly the Ballistic Coefficient is different also .269 for the 223 REM and .246 for the XM193.

Looks like the bullet for 223 REM is boat tail where as the M193 is not.  Can't see that adding almost 100 FPS though.

In my M-16A1 retro clone with 1/12 twist barrel Federal Red Box .223 55 grn averages 2996 fps out of the 20in bbl.

XM-193 ave. 3206 fps

IMI M-193 Ave. 3304 fps.

60 grain Nosler Partition handload Ave 3112

M-193 is a boat-tail spitzer bullet. Real M-193 that is. Don't know if Federal is substituting to keep up with demand or not.

5.56 is normally quoted from a 20" barrel since the old M-16s had 20s.

223 is normal rifle ammunition and it is tradition to calculate MV from a 24" barrel here.  Europe uses a 26" barrel I believe.

Yep.

Still that would put the 223 loading pretty much just as hot as M193.  Which does not make sense at all. Take away an assumed 4 inches of barrel for the 223 and it would likely just be down to the M193 velocity if even that low.  Something is not jiving here.
Like stated in the earlier post federal red box should be significantly slower than XM193 but federal does not claim that at all.

That just seems to run counter to everything previously known about 223 vs 5.56.  back in the days that everyone shot Winchester q3131 it was accepted that 5.56 was a hotter round than 223, now all the sudden they are the same?  When did this change?  At this point we are only assuming Federal used the longer rifle barrel for testing which I think would be the case but still the velocity for their 223 load seems at least equal if not hotter than 193.

Dont believe whats on the box.  Especially for cheap mass produced blasting ammo.  They are not going to change or reprint boxes if their specs change and specs change from year to year.  You will see that in some threads on here where people chrono the same ammo but made in two different years.  Most BCs will not be accurate either.

Hornady claims that their 50BMG ammo throws a 750gr AMAX at 2820 from a 24" barrel.  Thats possible at like 80KPSI.  Somebody was doing the printing, slapped 24" on the box, and moved on.  
Hornady 50BMG

Treat it like you would the MPG rating on the dealer sticker of a new car.  Its more suggestive of the performance than a hard and fast fact.

Bingo.
Errata: .223 Rem is typically tested out of a SAAMI 24" bbl.  Last acceptance spec I saw for M193 calls for 20" bbl, chrono at 25m from the muzzle.. So one must further extrapolate (with software making it easy).

Yesterday, out of my 16" mid-length 1:7, MV calculated after measurements adjusted for chrono distance by software:
"red box" AE223 - 3005
LC XM193 - 3103

Looks like one apple, one orange.

Ok so it's safe to say Federal 223 is not as hot as XM193 as we've always known.  I want to pick up a few boxes of 223 for an old school mini 14 that I've owned since the 80's.  It's ok for 5.56 but I only ever shot 223 and the barrel heats up very quickly.  I just think the 223 loading will be a little easier on the rifle.

Federal .223 = no crimped primer
LC (Federal) & all other 5.56 = crimped primer

Gots to be a good reason.

BTW, disappointing results today on reload testing.  Using all FNM cases, CCI 450's, and .223 load data in a 5.56 chamber, I had 2 fail to strip a cartridge from the mag, and 2 that did strip but failed to lock the hammer back..  Been a while since I loaded .223/5.56, so tested 3 different loads.  The 2 hotter loads were good, but at upper limits of .223 data.  The failures were middle of the road .223.  Should add this was full gas (adjustable) piston upper on a lower with an H2 buffer.  Suspected I might be light going in, and confirmed.

YMMV

In the Q3131 days we did not really talk about pressure.  We talked about velocity out of x length barrel and M193 in almost all cases would out pace 223 loadings like American Eagle.  So I guess what I am asking is has anyone put these loads over a chrono lately to see if that still holds true.  If 223 Rem FMJ loads are suddenly keeping pace with XM193 then something has changed somewhere.

Sorry, confused. I thought the comparison of American Eagle 223 FMJ with XM193 I gave (above) from a couple days ago would be something like what you were looking for.

They have not. Any .223 Remington load I've tested has never outpaced a 5.56x45mm loading.

It was! You were spot on and your findings are what we have come to expect.  It just sounded to me like some members posting in the thread are saying the loadings are comparable because of pressures and such.  I'm not saying they are wrong it's just not what has held true in the past.
I don't have a chrono right now to test myself.  It sounds like this is just a typical case of velocities printed on the ammo box being utterly inaccurate in actual practice.  You would think the ammo manufacturer could print something remotely close to reality on their packaging but they usually don't.  Thank you for posting your recent data.

Ok, excellent that's been my experience also.  I guess we can all return to our regularly scheduled program, lol.

I think the key words here are “known” and “accepted”. They are often interchangeable in usage (kinda like how you linked them in your post) even though everyone should know better than use them so.

From my understanding, the parameters for measuring velocity aren’t as rigid since SAAMI is worried about safety (mainly pressure). So for velocity, there can be quite a bit of variation in how the company measures it...different rifle actions, barrel lengths, even raw measurement  and averaging variations.

Now, imagine how a company that is trying to sell ammo to a public that believes 5.56 is hotter than .223 is going to approach marketing both their 5.56/.223 ammo. Do they want you to only buy 5.56 ammo for your AR? Or do they want you to buy their .223 ammo rather than a competitor’s 5.56 ammo? So for an AR owner, what is going to prompt you be comfortable buying .223?

And to further muddy the waters, they can’t really abandon the .223 label and label everything as 5.56, because then a bunch of bolt gun guys are going to abandon their product line for fear of blowing up their rifles with 5.56 ammo.

But, their lawyers also aren’t telling them they have to put warning labels on 5.56 ammo boxes about using it in .223 rifles, are they?

Just think about all the confusion over the years over .308 and 7.62NATO. I can see ammo makers basically throwing up their hands and trying to work within the buyers’ preconceived notions and prejudices rather than risking sales by trying to clarify and educate the buyers in limited space on product packaging.

Edit: a poster above implies that SAAMI specifies how velocity is measured. My understanding was that they don’t strictly regulate that. So I may be wrong.

Correct, also 5.56 velocity is measured at 78ft, and 223 velocity is measured at 15ft.

In addition to barrel length, commercial .223 chambers are going to have less freebore in them compared to 5.56 chambers.

This could certainly increase pressure in a .223 chamber.

Since there is a correlation between pressure and velocity, it may be that chamber dimensional differences between the test rifles are also affecting velocities in some way as well.

I know that I regularly get AE .223 ammo (per the package label) that is indistinguishable, even in the headstamp markings, from Federal 5.56 ammo (per the package label).

There is a lot of misinformation out there. But people who are actually testing things like empty case weight, brass thickness, etc. are finding no difference (once you take a large enough sample) in cases. I’ve come to the conclusion that there really isn’t a difference in the two outside of the specification features of the components, such as crimped primers, sealants, cannelures, flat base versus boat-tail, etc.

But if there’s doubt, someone should be able to simply chrono the various offerings that are labeled 5.56 compared to .223 from the same rifle on the same day to eliminate rifle differences and environmental conditions from the equation. I mean, I can walk into the local Cabela’s and they have both AE 5.56 and AE .223 on the shelf. It can’t really be that hard to put this to rest once and for all, can it?