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Thread: 5.56 vs 223 question

  1. #1
    Boolit Master
    Drm50's Avatar
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    5.56 vs 223 question

    Will someone who really knows the 5.56 vs 223 stuff please tell me the difference in the 2 prior to the Govt going to the heavier bullet and chamber dimensions. Iíve a 223 rifle that Iíve fired thousands of GI ball 55gr FMJ with no problems. I load 55gr Bullets at 3250fps with BLC2. Using RCBS 223 dies and both GI 5.56 and 223 brass. I started in 60s using IMR-4198. This brought to my attention that GI cases had less capacity than 223 brass. Iíve read countless articles on this and have not seen one plain explanation as to the early GI 55gr / commercial 223. My argument is there isnít any difference enough to worry about. The chamber pressure may differ a small amount but not enough to damage the gun. Iíve never found it necessary to run 223 heavier than 55gr/ 3250. Guys hot rodding with heavier bullets would increase pressure. I canít see where Iím off base on this. What I know comes from loading and shooting. Everyone tells me Iím wrong and ruining my rifle.

  2. #2
    Boolit Master trails4u's Avatar
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    Most of the past (maybe still current) confusion surrounding this comes from differences in the way pressure was measured. They are externally identical, and for all practical purposes can be considered identical. Yes....some brass will be thicker and have less capacity, but this is also true in commercial brass.
    "Do not follow where the path might lead, go instead where there is no path and leave a trail" Ralph Waldo Emerson

  3. #3
    Boolit Master
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    Biggest differences i found is the crimped primers. I’m by far not an expert and my experience is also loading and shooting over chronograph. I did find that the 5.56 has large jump of .045. Mine get set out to .010 in Thompson contender with very good accuracy out to 300 yards. Using AA2200 powder @ 24.5 grs.

  4. #4
    Boolit Master trails4u's Avatar
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    I suppose one caveat(potential pressure situation?) might be a really tight, short .223 chamber being fed 5.56 NATO that's on the hot end.
    "Do not follow where the path might lead, go instead where there is no path and leave a trail" Ralph Waldo Emerson

  5. #5
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    the 5.56 is a hotter load. a barrel marked 223 may or may not take the extra pressure. a barrel marked 5.56 can do everything just fine. supposedly the shoulders are a little different too.
    if you are ever being chased by a taxidermist, don't play dead

  6. #6
    Boolit Grand Master


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    Read my post #5 in https://castboolits.gunloads.com/sho...-223-Remington

    There you will see the only real difference is the pressure levels loaded to. Commercial ammunition and most current "223 Remington" load data adheres to SAAMI psi specification while military arsenals loading 5.56 do not. The 5.56 is, in fact, loaded to higher pressure. Also the difference is in the barrel twists and chambers. Barrels with 12 and 14" twists have/had chamber throats usually shorter for the M193/55 or less gr bullets used. Barrels with 7 - 10" twist have the longer throated chamber for the longer SS109/M855 bullets.

    As noted in the referenced post I use milspec loads with appropriate bullet weight/length for barrel twist in gas guns and bolt guns regardless whether marked "223" or "5.56". I use 223 Remington spec loads for break open actions. As like the OP I have been shooting thousands of M193 ammunition through numerous bolt action rifles with 12 and 14" twist barrels marked "223". There is no pressure problems with those actions as they are designed and made for magnum cartridge level pressures.

    I also have shot thousands of M193 5.56 NATO through ARs, M16s, M16A!s, M16A2sM4s, Mini14s and other military/civilian rifles with 7 - 10" twists w/o any pressure problems but sometimes a real degradation of accuracy with the faster twists. As to M855 with the 62 gr FMJBY or with match loads using 64 - 80 gr bullets the faster twist barrels of 7 - 9" are needed for accuracy whether a gas gun or a bolt match rifle. Those guns with faster twists are most often marked "223/5.56" and are made for milspec 5.56 pressures.
    Larry Gibson

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  7. #7
    Boolit Master
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    I will add that it always feels like there is a lot of trimming to get lc 556 brass down to 223 trim length. I’d be a little concerned about a 223 throat on the short end when shooting 556.

  8. #8
    Boolit Grand Master


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    Quote Originally Posted by JimB.. View Post
    I will add that it always feels like there is a lot of trimming to get lc 556 brass down to 223 trim length. I’d be a little concerned about a 223 throat on the short end when shooting 556.
    Many seem to perceive that as a "concern"........it's not. The psi's I listed in the mentioned post were all measured in a tight 223 Remington chamber with minimal throat.
    Larry Gibson

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  9. #9
    Boolit Master gnostic's Avatar
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    I've always thought the difference between 5.56mm and .223 cal, was 5.56mm is the metric description of .223 cal...

  10. #10
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    Couple of the better articles on this subject

    https://www.luckygunner.com/labs/5-56-vs-223/

    https://ultimatereloader.com/2018/08...cts-and-myths/

    Bottom line is don't shoot 64 grain tracers in a .223 chamber. Other than that it's a non-issue.
    Last edited by M-Tecs; 05-01-2021 at 03:17 PM.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

    "Before you argue with someone, ask yourself, is that person even mentally mature enough to grasp the concept of different perspectives? Because if not, thereís absolutely no point."
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  11. #11
    Boolit Master

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    The difference in the round is primarily one of higher chamber pressure in the 5.56.

    The difference in the GUNS is that 5.56 throats are longer to compensate for this.

    Whether 5.56 in a .223 chamber is going to cause problems gets you into "The Land of Variables" and "Each Rifle Is A Law Unto Itself". There's been enough tinkering with the platform for accuracy that it comes down to "WHICH .223 throat spec are you talking about?" or whether it's a bolt or gas gun. Also worth noting is that every .223 chamber will eventually burn out it's throat to 5.56 length and beyond.

    This is really no different than going too hot with your handloads to the point of flattening, piercing, or popping primers out of their pockets. The 5.56-into-.223 dilemma is that you're skipping the load workup and jumping straight to the point where some individual rifles will say "none of that for me, thanks".

    Consider the advisements the warning on the map that says "Beyond here, there be Dragons". You're getting beyond what the manufacturer spec'd the gun for constant use with, so they're basically washing their hands of the matter if you go up in a puff of smoke.
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  12. #12
    Boolit Bub
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    Several years ago, at a gun show, I watched a guy argue with a seller that the AR15 bolt for 5.56 was different than the .223. The seller could not dissuade him of his position. It was somewhere between sad and funny.

  13. #13
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    A lot of the 556 chambers out there have short 223 throats. Get a 556 throat gauge if you don't believe me. It used to be the throats on most 223 chambers would swallow anything. Notice how in the last 15 years or so most factory guns are shootIng under 1"? The throats have been tightened up a bunch.

  14. #14
    Boolit Master
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    Quote Originally Posted by Drm50 View Post
    Will someone who really knows the 5.56 vs 223 stuff please tell me the difference in the 2 prior to the Govt going to the heavier bullet and chamber dimensions.
    Originally the cartridges were to the same SAMMI specifications and only difference was the NATO name. SAMMI is only observed in U.S. commercial sporting guns and ammo while the military can do whatever they wish. In the military's serious need to field an effective battle rifle instead of a poodle shooter they needed a heavier projectile at the same or greater speed and that demanded more chamber pressure than SAAMI accepted; ergo, the 5.56 NATO cartridge was born again in the same cases.

    You have obviously proven that 5.56 ammo can safely be fired in a .223 rifle. The most reasonable fear is that the heavier gas impulse will subject the operating system to a greater beating than it was designed for.

    This brought to my attention that GI cases had less capacity than 223 brass.
    That's often true but not invariably; everything varies a bit!

    Iíve read countless articles on this and have not seen one plain explanation as to the early GI 55gr / commercial 223.
    That's because there was no difference at first.

    My argument is there isnít any difference enough to worry about. The chamber pressure may differ a small amount but not enough to damage the gun.
    Weelll ... semi/sorta. The chamber pressure difference is real but not vast. Thus, the damage is accumulative so there will be no one round KaBoom! And you could wear a barrel out before it fails. But what are you killing with your 5.56 ammo, why beat the snot outta your lower pressure operating system just to punch holes in paper????

    I canít see where Iím off base on this. What I know comes from loading and shooting. Everyone tells me Iím wrong and ruining my rifle.
    It's your rifle. And maybe they're wrong anyway ... maybe.
    Last edited by 1hole; 05-03-2021 at 11:44 AM.

  15. #15
    Boolit Grand Master


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    Read post #8 in; https://castboolits.gunloads.com/sho...-223-Remington

    Any increase in pressure between 223 Remington and 5.56 NATO is still well within the safe pressure parameters of any modern bolt action, gas gun or most SSs with SAAMI spec or military spec chambers. There will be no "accumulative damage". The OP doesn't say what action his rifle is but he is not wrong having "fired thousands of GI ball 55gr FMJ with no problems. I load 55gr Bullets at 3250fps with BLC2."
    Last edited by Larry Gibson; 05-03-2021 at 01:51 PM.
    Larry Gibson

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  16. #16
    Boolit Buddy

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    I agree with Larry. In a modern bolt action, there isn't a nickels worth of difference between the two chamberings. They are fully interchangeable.

  17. #17
    Boolit Master
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    Now I’m talking original 5.56 loading of 55gr FMJ , Only. All bets are off when you are talking heavier bullets, different twists ect. You never herd this controversy before military went to heavier loadings. I say flat out there is going to be no damage done to rifle shooting the old GI 55gr load at 3250fps. It doesn’t matter how you measure the pressure as long as both use same method. The only difference would be small pressure difference from small volume difference in cases. I still have a Ruger #3 / 223 that has fired thousands of GI 55gr loads. Had two other bolt 223s that are now owned by relatives and they show no signs of wear or damage.
    The “experts” won’t hit this head on, they act like they have a secret piece of information. I’ve yet to hear any that makes sense.

  18. #18
    Boolit Master
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    All my 223 shooting is on varmits. 55gr SP/ 3250fps. No semi autos. Bolts and Single Shots. The only time my rifles go on paper is to sight in.

  19. #19
    Boolit Master

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    You are correct - in a bolt gun, there is a whole lot less going on. You action is undoubtedly spec'd to take considerably more than anything that can reasonably be done in the .223 case. I've seen a 700 that in .308 that was mistakenly loaded with a full case of a fast pistol powder. The action was effectively destroyed, and major injury was avoided due to shooting glasses (liquefied brass sprayed back through the firing pin channel), but the action did not "let go" in a way that would have sent shrapnel to the four corners of the world.

    Loose primers rattling around inside an AR-15's upper and lower receiver have created havoc ranging from frozen operation to run-away full auto depending on where they land. This is why the authentic military 5.56 spec calls for crimping them in against the higher chamber pressure.

    Perhaps a bit outside the discussion of CHAMBER pressure, but one of the other concerns that happens in the wider AR-15 family begins when you get into the really short barrel lengths and their correspondingly shorter gas tubes. The size your gas port needs to be depends on how much barrel you have both behind and in front of the port. Short barrels have more pressure at the port and less barrel in front of the port creates a need to tap more gas more quickly. Consequently, the 10.5" "Commando" versions cycle pretty hard, necessitating heavier recoil buffers and generally higher levels of maintenance and parts replacement throughout. That gets worse on any of them when you mount suppressors, as that increases the amount of "dwell time" for gas to get forced through the port before the bullet completely leaves the system.

    Maybe I'm wrong, but I'm not aware of any change to the military CHAMBER dimensions. The major change was from the 1-12"
    twist of the M16-A1 to the 1-7" twist of the A2/A3/A4/M4 family.
    WWJMBD?

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  20. #20
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    Eugene Stoner originally designed the AR-15 with a 1:14 twist barrel, firing a 55gr bullet. The first Colt 601's were 14 twist. If I remember correctly winter testing showed instability issue and they switched to 12 twist. The 7 twist came with the adoption of the 62 grain NATO round.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

    "Before you argue with someone, ask yourself, is that person even mentally mature enough to grasp the concept of different perspectives? Because if not, thereís absolutely no point."
    Ė Amber Veal

    "The Highest form of ignorance is when your reject something you don't know anything about".
    - Wayne Dyer

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