I know we have all been down this road a time or 2 and it always comes up as 5.56 has higher pressure than the .223 and hotter charge than the .223 . I have done some searching on this as I find it kind of hard to believe since the velocity is really pretty equivalent between the 2 using the same grain bullet . From what I have read and I printed all these doncuments and have gone thru them several times to recheck myself . I think what people are seeing as pressure differences in specs are complete different types of pressure measurements . Such as the SAAMI rating , EPVAT rating , SCATP5.56 rating and the CIP ratings .
Heres what I dont get . I have copied and pasted this for reference
The US military defines test procedures for 5.56mm NATO in SCATP 5.56, and 7.62mm NATO in SCATP 7.62. These procedures are based on the SAAMI test methodology.
NATO, however, defines 5.56mm, 7.62mm, 9mm and 12.7mm using the NATO EPVAT test methods, which includes pressure testing that generally conforms to C.I.P. pressure test methods instead. For the 9mm NATO EPVAT specifies that for 9×19mm Parabellum (9mm Luger in C.I.P. nomenclature and 9mm NATO in NATO nomenclature), the transducer must be positioned at the mid case position (9.5 millimetres (0.37 in)) from the breech face instead of C.I.P.'s 12.5 millimetres (0.49 in) from the breech face. For NATO EPVAT testing of military firearms ammunition NATO design EPVAT test barrels with Kistler 6215 channel sensor transducers are used.
Yet the EPVAT maximum pressure rating on the 5.56X45 cartridge is 62,366 psi or 430.00MPa and the CIP maximum pressure for the .223 is 62,366 psi or 430.00 MPa .
I know that the specs for the 5.56 brass are basically the same with the exception of the radius on the neck on the 5.56 its squared off on the .223 its radiused 2,54 at the neck and ,64 on the shoulder .
Im just wondering how much more pressure there really is shooting 5.56 in a .223 chamber with the different radius on the cartridge .
Much more research led me to this link http://www.luckygunner.com/labs/5-56-vs-223/#bookmark4
which was very interesting and led me to believe that there are increased pressure because of the tighter tolerances of the .223 over the 5.56 , not the fact that the 5.56 is a hotter cartridge than the .223 .
In reality is your rifle going to blow up , very highly unlikely . Is it harder , more wear and tear on your rifle ? Not really . Is it really unsafe to fire 5.56 in your .223 chamber ? Thats a answer you have to ask your self .
Have I done it ? Yes. Is there a noticeable difference to me ? No . Chronograph differences were so minor it could very between different cartridges of the same ammo . Most differences were less than 25 fps even with the same brand cartridges I used . This shows me that factory ammo #1 isnt real consistent and #2 will NEVER be as accurate and the guy loading precision reloads .
In a SHTF scenerio , I would put either thru it without second guessing myself on it . IMO your going to get better accuracy our of the tighter tolerances of the .223 than you are a 5.56 round
5.56 on the top , .223 on the bottom