223 vs 5.56

gerard488 · February 4, 2014, 01:12 PM

I posted this question on the savage shooters forum but it was shut down by the moderator so here goes:
How many people have had a problem because of shooting 5.56 nato ammo in their .223 ?
Not looking for opinions or speculation, just the cold hard facts
Looking for a first hand account of an actual problem.
I hear some stories about a cousins brothers neighbours uncles best friends daughters boyfriends brother blowing up his barrel but I would like to find if there has ever been an actual problem.
I`m looking for a firsthand account of actual damage to a rifle barrel, bolt, chamber, action or injury to a shooter or bystander
I don`t mean to start a discussion or argument about firearm safety, gun manufacturer policies or warranties, case dimensions, chamber pressures or accuracy. Just looking for an actual account of damage from someone who saw it with his/her own eyes.

Jimro · February 4, 2014, 02:07 PM

No issues with my Colt HBAR upper, labeled 223 but was told that all Colt's have 5.56 chambers regardless of barrel stamp, have no idea if that is true or not but I haven't had any issues.

If you put a round in the chamber, let the bolt slam forward, then withdraw the round and you don't see rifling marks on the bullet or have a bullet stuck in the throat then you should be fine shooting that ammunition through your rifle.

The tolerances for 223 chambers overlap with 5.56 chambers, which is why some AR shooters like myself don't need to use small base dies to reload, and other AR shooters do.

Jimro

Sharkbite · February 4, 2014, 02:13 PM

The chambers are the same. The diff is in the length of the throat. A 556 barrel has a longer throat or leade. This allow the higher pressure NATO round to be used.

Ive seen PLENTY of guns marked 556 that actually measure as .223

The result of shooting true 5.56 in a true .223 chamber is increased pressure. This CAN result in blown primers. I have seen primers end up in the trigger group and tie up the gun.

Dc777 · February 4, 2014, 02:22 PM

Hasn't this been discussed multiple times on here? I remember seeing a thread about it a couple of weeks ago. Use the " search " feature.

Art Eatman · February 4, 2014, 02:47 PM

I can't recall ever reading about a bolt-action .223 having a problem with GI 5.56. Hasn't hurt mine, anyhow.

Any chamber pressure difference between the two is due to whatever pressure measuring system was used. PSI vs. CUP. They aren't the same. Me, I really doubt that GI stuff is loaded "hotter".

Jimro · February 4, 2014, 03:02 PM

Art Eatman,

M855A1 and Mk262 Mod 0/1 are both spec'd to 62k psi IIRC. They have to be to get the desired velocities from M4 length barrels. The 55k psi limit for 223 is way on the conservative side.

That being said, there shouldn't be a problem using them in 223 chambers as they are still signicantly below a "proof load" pressure level.

Jimro

jmr40 · February 4, 2014, 03:09 PM

Same ammo, different names. Military 5.56 stamped rifles have slightly looser tolerences for more reliable feeding than SOME 223 chambered rifles. It is possible that SOME military 5.56 ammo is loaded slightly hotter than SOME 223 ammo. The reality is that most guns and ammo overlap. No 2 manufacturers load exactly the same and you will find pretty wide variances in guns chambers from one manufacturer to another in any chambering.

In the extremely rare possibility you end up with some of the hotter 5.56 ammo in a 223 chamber cut to minimum dimensions you MIGHT have some sticky bolt lift on a bolt rifle and unreliable feeding in a semi-auto.

But the odds of that happening with any other chambering are just as good. I've lost count of how many threads we have had here where someone purchased a bolt rifle in 7mm mag, 30-06, 243, 308 etc. and ran into a particular brand of factory ammo that was just too hot in that rifle and had the same issues.

This is very common with factory loaded 30-06 ammo. There are at least 3 very different power levels of ammo and far worse issues could arise if the wrong ammo is used in the wrong guns. No one seems to care and we don't even have 2-3 different names for the different loadings. It is all just 30-06.

Sure Shot Mc Gee · February 4, 2014, 06:08 PM

I don't believe I've ever shot any Nato 556 out of my Colt. Just Federal American Eagle & Premium and my reloads. I always thought 556 is basically NATO military surplus. Being it's made around the world and marketed here by what-ever importers. If you incur a problem with its use. Where's their Customers Service Dept. Kyrgyzstan maybe. That's why I'm not comfortable with using it.

71Eagle · February 5, 2014, 07:19 PM

I have a Remington 700 VLS in .223. I called Remington and they said do not fire a 5.56 out of my rifle. It will void any warranty.

Sharkbite · February 5, 2014, 07:47 PM

Guys... Shooting 5.56 ammo out of a .223 gun is a bad idea. The shorter throat on the 223 gun WILL cause a higher pressure spike at the begining of the curve.

Hence the above post with Remington talking about voided warranties.

Is it likely to result in a damaged/blown up gun?... No.
Does it cause all the associated problems of firing over pressure ammo?... Yes

I have seen first hand... guns blow primers due to this

4runnerman · February 5, 2014, 07:57 PM

Sharkbite-- This is what he is looking for- Tell us about it. I know about 7 people that do, Never a issue.

Jimro · February 5, 2014, 08:06 PM

Sharkbite, milspec 5.56 ammo should have crimped primers to prevent primer blow out. Do you happen to remember what 5.56 load your friends were using? American milsurp or imported?

Jimro

Sharkbite · February 5, 2014, 08:25 PM

Commercial AR's that had the barrel stamped 5.56. Shooting commercial ammo (not mil-surplus) again marked 5.56. I forget which brand

The guy running the gun said he had that happen a few times before the class

After about the 2nd primer ending up in the lower. We put a 5.56 throat gauge into the gun... No go. It had a 223 leade barrel

Again. Nothing catastrophic. Just tied up the gun until we got the primer from inside the trigger works

The take home msg is that is a classic sign of over pressure. I have every AR i buy checked for correct leade. If it says 5.56 it better be 5.56

**JohnKSa** · February 5, 2014, 09:11 PM

A pretty thorough treatment of this topic may be found below.

http://thefiringline.com/forums/showthread.php?t=530912

It's not that hard to find folks with firsthand experience of the mismatch causing issues. Blown primers is the most commonly recounted issue, and, not coincidentally that's what the experts advise is the most likely problem to be encountered.

Quote:

I know about 7 people that do, Never a issue.

Sure--if you look at the differences in the two cartridges/chambers, it's pretty clear that the differences aren't huge. Patrick Sweeney indicated in one of his articles that he saw most of the problems during warm weather. The little extra pressure boost from the higher temperatures was just enough to push things over the edge.

TMW89 · February 5, 2014, 09:44 PM

I always thought you could shoot .223 out of a 5.56, but not the other way around.

**JohnKSa** · February 5, 2014, 09:48 PM

That's the one sentence summary.

Jimro · February 5, 2014, 09:49 PM

Getting back to sharkbite's data point, I'd like to point out that other factors can explain popped primers.

Not all "5.56x45" ammunition on the market is created equal. http://www.luckygunner.com/labs/5-56-vs-223/

If you read through that link, you'll see a "secondary pressure spike" happening, and I think more often than not that is the real culprit behind "don't shoot 5.56 in a 223 chamber!" bit of advice comes from.

Secondary Pressure Spike explained in much further detail here, http://www.shootingsoftware.com/barrel.htm

But I'll fall back on my earlier advice. Chamber the ammo, then pull it out. If there are no marks on the bullet and you have a clear bore, there is nothing wrong with the dimensions of that ammo in your chamber. That doesn't tell you anything about the powder charge though.

Jimro

**JohnKSa** · February 5, 2014, 10:00 PM

Quote:

...I'd like to point out that other factors can explain popped primers.

Of course.

BUT, when you're doing A and having problems with B and the experts tell you that doing A can cause problems with B and there are other examples of people doing A and having problems with B, Occam's Razor indicates that looking for some other factor besides A to explain B isn't a wise approach.

The luckygunner article is pretty good. He covers the topic very thoroughly but many people read it and only see what they want to see. A close read indicates that he has seen problems with shooting 5.56 in a .223 chamber. What problems, you ask? Blown primers. Imagine that.

Jimro · February 5, 2014, 10:18 PM

JohnKSa,

The question isn't that the primers were blown, the question is WHY?

Was it the chamber, or was it a secondary pressure spike?

Seriously, the article linked here (from the previous thread): http://www.murraysguns.com/223vs556.htm lists 77k psi as the result of 5.56 ammo in a 223 chamber.

Unfortunately 77k psi won't pop the primers out. Case head expansion doesn't start until 85k psi according to Clark, who has tested more cartridges to failure than I care to count (and I believe him on this), and primers don't pop out until pressures are higher than that.

So, is it the chamber? Which I think if you look at the luckygunner data shows that the first pressure spike never went over 70k, or a secondary pressure spike where pressures could spike over 120k as outlined in the second URL I previously posted?

I guess it could be some weird combination of both, as there is no evidence of mutual exclusivity.

Jimro

**JohnKSa** · February 6, 2014, 12:07 AM

Quote:

...the question is WHY?

I understand the question.

Whether or not we are able to pinpoint the precise mechanism by which the mismatch causes trouble, it does seem pretty clear that it can cause trouble which is what the OP wanted to know.

It's not likely to blow up a gun, but it can cause problems which are serious enough that one would think any reasonable person would avoid them if possible.

Quote:

...primers don't pop out until pressures are higher than that.

I think that's why so many people get away with the mismatch. What the mismatch does is set the stage for another factor, one that would normally be totally harmless, to push things past the breaking point. Sweeney's experiences in warm weather support that explanation.

And, it's worth noting that even if, for the sake of argument, we accept that 77Kpsi is not sufficient pressure to cause blown primers, in and of itself, it is still 40% higher the SAAMI maximum peak pressure specification for .223 Remington. That's well above even proof pressure.

bamaranger · February 6, 2014, 01:46 AM

I ran some NATO marked 55 gr FMJ thru my Mini-Mauser this summer and chrono'd the shots, the average for the string was 3950 fps +/-. I don't have the notes in front of me, but that was several 100 fps faster than sporting 55 gr SP's. Believe all ammo was W-W.

No readily apparent pressure signs, but the wide variation and high velocity has convinced me not to to it again. The NATO stuff didn't group so hot either, 3MOA from a rifle that can shoot 1 MOA +/- with the right stuff.

TMD · February 6, 2014, 08:27 AM

It was never an issue before the internet was around.

Panfisher · February 6, 2014, 08:42 AM

So, some rifles marked .223 Rem are actually 5.56 and even some marked 5.56 are actually .223 Rem?. Ain't nothing sacred anymore. Maybe just maybe some of the 5.56 marked ammo is actually .223 and versa visa.

Jimro · February 6, 2014, 02:30 PM

JohnSKa,

In the linked luckygunner article, the secondary pressure spike was seen with both Tula and Silver Bear ammunition out of 5.56 chambers. To me that indicates and ammo problem (specifically a powder problem) and not a chamber problem.

From what I can tell, SAAMI proof loads for 223 Rem (based on the 55k pressure standard) is 69.5k to 74.5k CUP. So 77k CUP is over a proof load.

The CIP standard of 62k, if held to the same 1.3x to 1.4x operating pressure would be 80.6 kpsi to 86.8k psi. Here 77kpsi is under a proof load.

Just a reminder that Eurospec 223 Rem is not the same as SAAMI spec 223 Rem, and shooting the M855 5.56 ammo through the 223 chamber didn't push the max pressure over 62kpsi in the luckygunner test, so the ammo would still qualify as "223 Rem" in CIP specification.

What I think is happening is that the secondary pressure spike is what is popping primers, I can't prove it, but based on actual pressure trace data we have available (instead of anecdotal "the sun was out and I started popping primers, must be that 223 chamber of mine!") the primary pressure spike is not responsible for popping primers.

So, back to Occam's razor based on the available evidence, what say you?

Jimro

SaxonPig · February 6, 2014, 03:34 PM

I have an old (1978) Colt AR 15 and two "Mini Mauser" bolt guns in 223. I have used military stamped 5.56 and commercial 223 Remington ammo in all three and none of the rifles seems to care which ammo it is being fed. I have not noticed any pressure signs with any ammo in any gun.

I keep hearing they are not the same cartridge. I have heard the chamber is a little different between 5.56 and 223. I have heard the shoulder is slightly forward on 5.56 (for full auto use). I have heard they are loaded to different pressures. But when I compare them I see no difference. When I shoot them I see no difference. Maybe I am missing something?

February 4, 2014, 01:12 PM	#1
gerard488 Junior Member Join Date: August 12, 2013 Posts: 5	223 vs 5.56 I posted this question on the savage shooters forum but it was shut down by the moderator so here goes: How many people have had a problem because of shooting 5.56 nato ammo in their .223 ? Not looking for opinions or speculation, just the cold hard facts Looking for a first hand account of an actual problem. I hear some stories about a cousins brothers neighbours uncles best friends daughters boyfriends brother blowing up his barrel but I would like to find if there has ever been an actual problem. I`m looking for a firsthand account of actual damage to a rifle barrel, bolt, chamber, action or injury to a shooter or bystander I don`t mean to start a discussion or argument about firearm safety, gun manufacturer policies or warranties, case dimensions, chamber pressures or accuracy. Just looking for an actual account of damage from someone who saw it with his/her own eyes.

February 4, 2014, 02:07 PM	#2
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	No issues with my Colt HBAR upper, labeled 223 but was told that all Colt's have 5.56 chambers regardless of barrel stamp, have no idea if that is true or not but I haven't had any issues. If you put a round in the chamber, let the bolt slam forward, then withdraw the round and you don't see rifling marks on the bullet or have a bullet stuck in the throat then you should be fine shooting that ammunition through your rifle. The tolerances for 223 chambers overlap with 5.56 chambers, which is why some AR shooters like myself don't need to use small base dies to reload, and other AR shooters do. Jimro __________________ Machine guns are awesome until you have to carry one.

February 4, 2014, 02:22 PM	#4
Dc777 Senior Member Join Date: November 18, 2013 Location: South West, Alabama Posts: 609	Hasn't this been discussed multiple times on here? I remember seeing a thread about it a couple of weeks ago. Use the " search " feature. __________________ NRA member Paintings were made to look at, and guns were made to be shot. If you aren't going to use them as they are intended for, get rid of them and buy a Picasso.

February 4, 2014, 03:02 PM	#6
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	Art Eatman, M855A1 and Mk262 Mod 0/1 are both spec'd to 62k psi IIRC. They have to be to get the desired velocities from M4 length barrels. The 55k psi limit for 223 is way on the conservative side. That being said, there shouldn't be a problem using them in 223 chambers as they are still signicantly below a "proof load" pressure level. Jimro __________________ Machine guns are awesome until you have to carry one.

February 5, 2014, 07:19 PM	#9
71Eagle Member Join Date: June 20, 2011 Location: Central Texas Posts: 37	5.56/.223 I have a Remington 700 VLS in .223. I called Remington and they said do not fire a 5.56 out of my rifle. It will void any warranty.

February 4, 2014, 02:13 PM	#3
Sharkbite Senior Member Join Date: November 4, 2013 Location: Western slope of Colorado Posts: 3,679	The chambers are the same. The diff is in the length of the throat. A 556 barrel has a longer throat or leade. This allow the higher pressure NATO round to be used. Ive seen PLENTY of guns marked 556 that actually measure as .223 The result of shooting true 5.56 in a true .223 chamber is increased pressure. This CAN result in blown primers. I have seen primers end up in the trigger group and tie up the gun.

February 4, 2014, 02:47 PM	#5
Art Eatman Staff in Memoriam Join Date: November 13, 1998 Location: Terlingua, TX; Thomasville, GA Posts: 24,798	I can't recall ever reading about a bolt-action .223 having a problem with GI 5.56. Hasn't hurt mine, anyhow. Any chamber pressure difference between the two is due to whatever pressure measuring system was used. PSI vs. CUP. They aren't the same. Me, I really doubt that GI stuff is loaded "hotter".

February 4, 2014, 03:09 PM	#7
jmr40 Senior Member Join Date: June 15, 2008 Location: Georgia Posts: 10,809	Same ammo, different names. Military 5.56 stamped rifles have slightly looser tolerences for more reliable feeding than SOME 223 chambered rifles. It is possible that SOME military 5.56 ammo is loaded slightly hotter than SOME 223 ammo. The reality is that most guns and ammo overlap. No 2 manufacturers load exactly the same and you will find pretty wide variances in guns chambers from one manufacturer to another in any chambering. In the extremely rare possibility you end up with some of the hotter 5.56 ammo in a 223 chamber cut to minimum dimensions you MIGHT have some sticky bolt lift on a bolt rifle and unreliable feeding in a semi-auto. But the odds of that happening with any other chambering are just as good. I've lost count of how many threads we have had here where someone purchased a bolt rifle in 7mm mag, 30-06, 243, 308 etc. and ran into a particular brand of factory ammo that was just too hot in that rifle and had the same issues. This is very common with factory loaded 30-06 ammo. There are at least 3 very different power levels of ammo and far worse issues could arise if the wrong ammo is used in the wrong guns. No one seems to care and we don't even have 2-3 different names for the different loadings. It is all just 30-06.

February 4, 2014, 06:08 PM	#8
Sure Shot Mc Gee Senior Member Join Date: January 2, 2012 Location: Minnesota Posts: 3,876	I don't believe I've ever shot any Nato 556 out of my Colt. Just Federal American Eagle & Premium and my reloads. I always thought 556 is basically NATO military surplus. Being it's made around the world and marketed here by what-ever importers. If you incur a problem with its use. Where's their Customers Service Dept. Kyrgyzstan maybe. That's why I'm not comfortable with using it.

February 5, 2014, 07:47 PM	#10
Sharkbite Senior Member Join Date: November 4, 2013 Location: Western slope of Colorado Posts: 3,679	Bad idea boys.... Guys... Shooting 5.56 ammo out of a .223 gun is a bad idea. The shorter throat on the 223 gun WILL cause a higher pressure spike at the begining of the curve. Hence the above post with Remington talking about voided warranties. Is it likely to result in a damaged/blown up gun?... No. Does it cause all the associated problems of firing over pressure ammo?... Yes I have seen first hand... guns blow primers due to this

February 5, 2014, 07:57 PM	#11
4runnerman Senior Member Join Date: January 16, 2010 Location: Minnesota Posts: 3,577	Sharkbite-- This is what he is looking for- Tell us about it. I know about 7 people that do, Never a issue. __________________ NRA Certified RSO NwCP- Performance Isn't Optional

February 5, 2014, 08:06 PM	#12
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	Sharkbite, milspec 5.56 ammo should have crimped primers to prevent primer blow out. Do you happen to remember what 5.56 load your friends were using? American milsurp or imported? Jimro __________________ Machine guns are awesome until you have to carry one.

February 5, 2014, 08:25 PM	#13
Sharkbite Senior Member Join Date: November 4, 2013 Location: Western slope of Colorado Posts: 3,679	Commercial AR's that had the barrel stamped 5.56. Shooting commercial ammo (not mil-surplus) again marked 5.56. I forget which brand The guy running the gun said he had that happen a few times before the class After about the 2nd primer ending up in the lower. We put a 5.56 throat gauge into the gun... No go. It had a 223 leade barrel Again. Nothing catastrophic. Just tied up the gun until we got the primer from inside the trigger works The take home msg is that is a classic sign of over pressure. I have every AR i buy checked for correct leade. If it says 5.56 it better be 5.56

February 5, 2014, 09:44 PM	#15
TMW89 Senior Member Join Date: March 3, 2013 Location: MI Posts: 185	I always thought you could shoot .223 out of a 5.56, but not the other way around. __________________ Springfield XDM9C Savage Mark II F .22 H&R ultra slug hunter 20 ga 1942 Mosin Nagant "Natalia"

February 5, 2014, 09:48 PM	#16
JohnKSa Staff Join Date: February 12, 2001 Location: DFW Area Posts: 24,992	That's the one sentence summary. __________________ Do you know about the TEXAS State Rifle Association?

February 5, 2014, 09:49 PM	#17
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	Getting back to sharkbite's data point, I'd like to point out that other factors can explain popped primers. Not all "5.56x45" ammunition on the market is created equal. http://www.luckygunner.com/labs/5-56-vs-223/ If you read through that link, you'll see a "secondary pressure spike" happening, and I think more often than not that is the real culprit behind "don't shoot 5.56 in a 223 chamber!" bit of advice comes from. Secondary Pressure Spike explained in much further detail here, http://www.shootingsoftware.com/barrel.htm But I'll fall back on my earlier advice. Chamber the ammo, then pull it out. If there are no marks on the bullet and you have a clear bore, there is nothing wrong with the dimensions of that ammo in your chamber. That doesn't tell you anything about the powder charge though. Jimro __________________ Machine guns are awesome until you have to carry one.

February 5, 2014, 10:18 PM	#19
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	JohnKSa, The question isn't that the primers were blown, the question is WHY? Was it the chamber, or was it a secondary pressure spike? Seriously, the article linked here (from the previous thread): http://www.murraysguns.com/223vs556.htm lists 77k psi as the result of 5.56 ammo in a 223 chamber. Unfortunately 77k psi won't pop the primers out. Case head expansion doesn't start until 85k psi according to Clark, who has tested more cartridges to failure than I care to count (and I believe him on this), and primers don't pop out until pressures are higher than that. So, is it the chamber? Which I think if you look at the luckygunner data shows that the first pressure spike never went over 70k, or a secondary pressure spike where pressures could spike over 120k as outlined in the second URL I previously posted? I guess it could be some weird combination of both, as there is no evidence of mutual exclusivity. Jimro __________________ Machine guns are awesome until you have to carry one.

February 6, 2014, 01:46 AM	#21
bamaranger Senior Member Join Date: October 9, 2009 Location: North Alabama Posts: 8,312	mini mauser I ran some NATO marked 55 gr FMJ thru my Mini-Mauser this summer and chrono'd the shots, the average for the string was 3950 fps +/-. I don't have the notes in front of me, but that was several 100 fps faster than sporting 55 gr SP's. Believe all ammo was W-W. No readily apparent pressure signs, but the wide variation and high velocity has convinced me not to to it again. The NATO stuff didn't group so hot either, 3MOA from a rifle that can shoot 1 MOA +/- with the right stuff.

February 6, 2014, 08:27 AM	#22
TMD Senior Member Join Date: August 9, 2011 Posts: 1,293	It was never an issue before the internet was around.

February 6, 2014, 08:42 AM	#23
Panfisher Senior Member Join Date: July 30, 2010 Location: Missouri Posts: 1,337	So, some rifles marked .223 Rem are actually 5.56 and even some marked 5.56 are actually .223 Rem?. Ain't nothing sacred anymore. Maybe just maybe some of the 5.56 marked ammo is actually .223 and versa visa. Last edited by Panfisher; February 6, 2014 at 09:19 AM.

February 6, 2014, 02:30 PM	#24
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	JohnSKa, In the linked luckygunner article, the secondary pressure spike was seen with both Tula and Silver Bear ammunition out of 5.56 chambers. To me that indicates and ammo problem (specifically a powder problem) and not a chamber problem. From what I can tell, SAAMI proof loads for 223 Rem (based on the 55k pressure standard) is 69.5k to 74.5k CUP. So 77k CUP is over a proof load. The CIP standard of 62k, if held to the same 1.3x to 1.4x operating pressure would be 80.6 kpsi to 86.8k psi. Here 77kpsi is under a proof load. Just a reminder that Eurospec 223 Rem is not the same as SAAMI spec 223 Rem, and shooting the M855 5.56 ammo through the 223 chamber didn't push the max pressure over 62kpsi in the luckygunner test, so the ammo would still qualify as "223 Rem" in CIP specification. What I think is happening is that the secondary pressure spike is what is popping primers, I can't prove it, but based on actual pressure trace data we have available (instead of anecdotal "the sun was out and I started popping primers, must be that 223 chamber of mine!") the primary pressure spike is not responsible for popping primers. So, back to Occam's razor based on the available evidence, what say you? Jimro __________________ Machine guns are awesome until you have to carry one.

February 6, 2014, 03:34 PM	#25
SaxonPig Senior Member Join Date: April 24, 2006 Posts: 1,900	I have an old (1978) Colt AR 15 and two "Mini Mauser" bolt guns in 223. I have used military stamped 5.56 and commercial 223 Remington ammo in all three and none of the rifles seems to care which ammo it is being fed. I have not noticed any pressure signs with any ammo in any gun. I keep hearing they are not the same cartridge. I have heard the chamber is a little different between 5.56 and 223. I have heard the shoulder is slightly forward on 5.56 (for full auto use). I have heard they are loaded to different pressures. But when I compare them I see no difference. When I shoot them I see no difference. Maybe I am missing something?