.223/5.56 & .308/7.62 chamber pressure difference questions

Kimio · July 30, 2014, 03:13 PM

Okay, so for the longest time, I've been told that you can use .223 ammunition in a 5.56 chambered rifle and 7.62 in a rifle chambered in .308

However you cannot do the reverse due to differences in chamber pressures caused by small variances in case dimensions.

For the most part, the .223/5.56 statement has been affirmed as being factual. However, while browsing another forum (namely the FAL Files) I came a across a thread that stated the interchangeability of the .308/7.62 is a myth or flat out incorrect. According to said thread, the two rounds may be shot out the rifle with no worries of safety issues or damage to the firearm.

I'm trying to discern if this is true or not. Can someone here who reloads please clarify if this is indeed fact, or just another research job that I must go through in order to separate the wheat from the chaff in regards to information

chiefr · July 30, 2014, 05:03 PM

I see no sense in continuing the argument, but I reload ammo for AR rifles identical as mil spec as possible.
Conversely, in my .223 bolt gun, I load to near max and with heavier bullets.
Same with 308. If the gun is mil, I load to mil specs.
One of the advantages of reloading is you can have it your way.

Bart B. · July 30, 2014, 05:26 PM

There may be issues with the 22 caliber cartridges but not with the 30 caliber ones. The 22 caliber case and chamber differences can be a problem, but rarely is. SAAMI mentions this in their document:

http://www.saami.org/specifications_...wnload/206.pdf

where you can also see the pressure system number differences between cup and psi values for all sorts of cartridges.

Frankford Arsenal set peak pressure limits for the 7.62 NATO round at 50,000 cup; same as the .30-06. Winchester set their commercial version called the .308 Winchester to 52,000 cup but their crusher gauge system was a bit different than the arsenals. They are the same for all practical and use purposes. Both can be shot in each others chamber rifles and have been done so for decades. But the arsenal's listed peak pressure in pounds per square inch, psi, which was and is a misnomer but traditional for decades using the copper crusher system.

The problem arose when strain gauges were used and measured in pounds per square inch and used different systems in the late 1960's. Both the 30 caliber cartrigdes were spec'd to about 61,000 psi. When someone saw the old system stating 52,000 psi for the service round and 61,000 psi for the commercial one, they got excited and claimed the commercial round would blow up a service rifle. Totally ignorant of the fact that thousands of rounds of commercial match ammo were shot in military match rifles without any issue whatsoever.

Same issue is with .30-06 arsenal ammo listed in 50,000 psi pressure compared to commercial .30-06 ammo listed in 61,000 psi pressure, but few people are aware of that.

So, moderators, you might want to lock this thread up before it goes viral or some other time wasting tyrade that gets way out of hand.

Brian Pfleuger · July 30, 2014, 05:53 PM

Quote:

Originally Posted by Bart B.

So, moderators, you might want to lock this thread up before it goes viral or some other time wasting tyrade that gets way out of hand.

Trust me, we're watching.

steve4102 · July 30, 2014, 06:29 PM

The 7.62 and the 308 run at the same pressures, or close.

Unclenick has posted on here a few times as to the differences or lack there of.

This is a quote from one of his previous posts. Sorry no link.

If you look through military ammo specs from before the mid-90's, you find military copper crusher numbers reported as "psi" at the same time a commercial maker would have reported them as "CUP" (I don't know why "CUP" gets caps while "psi" gets lower case, but that's how SAAMI publishes them). The SAAMI conformal piezo spec is 62,000 psi MAP (Maximum Average Pressure). I'll have to check, but I recall SAAMI MAP allows up to 4% deviation from average, so 64,480 psi would be the allowed extreme spread in SAAMI compliant ammo.

The NATO allies use 415 MPa, which the CIP uses for both 7.62 NATO and .308 Winchester. That converts to 60,191 psi, but the CIP measuring system sets the sample point further forward in the case, so it sees a bit of the pressure drop at the bullet base that occurs when the bullet starts moving. That makes them tend to read about 2,000 psi lower than the SAAMI setup does in this pressure range. So the same ammo would measure closer to 62,000 psi in SAAMI equipment.

You can also check out Commercial 308 and 7.62 ammo, here

http://www.midwayusa.com/product/953...oint-boat-tail

http://www.midwayusa.com/product/953...open-tip-match

Both have identical velocities, if they differed in pressure by the claimed 12K then velocities would also be reduced.

As this is a reloading forum, check out the 7.62 load data. Not the data that is rifle specific, data that is chamber specific. Guess what you can't, as there is no data that I am aware of that is specific to the 7.62 Chamber.

With thousand/millions of rifles chambered in 7.62 if they required their own data, you would think one or two of the powder manufacturers would cater to the 7.62 round, but they do not. Why? Because it is a 308 Winchester.

Now when using 7.62 Military Brass be aware that you may have to adjust your load due to reduced case capacity to stay within safe pressures for both the 308 and the 7.62.

You can also check this out.

http://how-i-did-it.org/762vs308/chamber.html

Kimio · July 30, 2014, 07:09 PM

Thank you for the replies and the resources. This helps clear a lot of things up.

Jimro · August 4, 2014, 07:06 AM

Milspec ammo must also meet port pressure requirements. For years folks knew that 30-06 was 30-06, but that the Garand shouldn't be fed just any random hunting ammunition. Same for the M1A/M14.

The DI system for the AR family is a little more forgiving, but still it is a consideration.

And completely aside from the calculated pressure of a particular powder charge you run into Secondary Pressure Spikes with some primer/powder combinations which can give you the same symptoms as an overpressure event. This seems to happen more with ball powder than extruded.

As far as pressures and dimensions go, I interchange 5.56/223 and 7.62/308 all the time and think nothing of it. Do a standard load workup to ensure it is safe in your rifle than load up a lot of ammo and have fun.

Jimro

July 30, 2014, 03:13 PM	#1
Kimio Senior Member Join Date: January 2, 2011 Location: Arizona Posts: 1,171	.223/5.56 & .308/7.62 chamber pressure difference questions Okay, so for the longest time, I've been told that you can use .223 ammunition in a 5.56 chambered rifle and 7.62 in a rifle chambered in .308 However you cannot do the reverse due to differences in chamber pressures caused by small variances in case dimensions. For the most part, the .223/5.56 statement has been affirmed as being factual. However, while browsing another forum (namely the FAL Files) I came a across a thread that stated the interchangeability of the .308/7.62 is a myth or flat out incorrect. According to said thread, the two rounds may be shot out the rifle with no worries of safety issues or damage to the firearm. I'm trying to discern if this is true or not. Can someone here who reloads please clarify if this is indeed fact, or just another research job that I must go through in order to separate the wheat from the chaff in regards to information Last edited by Kimio; July 30, 2014 at 07:05 PM.

July 30, 2014, 05:26 PM	#3
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	There may be issues with the 22 caliber cartridges but not with the 30 caliber ones. The 22 caliber case and chamber differences can be a problem, but rarely is. SAAMI mentions this in their document: http://www.saami.org/specifications_...wnload/206.pdf where you can also see the pressure system number differences between cup and psi values for all sorts of cartridges. Frankford Arsenal set peak pressure limits for the 7.62 NATO round at 50,000 cup; same as the .30-06. Winchester set their commercial version called the .308 Winchester to 52,000 cup but their crusher gauge system was a bit different than the arsenals. They are the same for all practical and use purposes. Both can be shot in each others chamber rifles and have been done so for decades. But the arsenal's listed peak pressure in pounds per square inch, psi, which was and is a misnomer but traditional for decades using the copper crusher system. The problem arose when strain gauges were used and measured in pounds per square inch and used different systems in the late 1960's. Both the 30 caliber cartrigdes were spec'd to about 61,000 psi. When someone saw the old system stating 52,000 psi for the service round and 61,000 psi for the commercial one, they got excited and claimed the commercial round would blow up a service rifle. Totally ignorant of the fact that thousands of rounds of commercial match ammo were shot in military match rifles without any issue whatsoever. Same issue is with .30-06 arsenal ammo listed in 50,000 psi pressure compared to commercial .30-06 ammo listed in 61,000 psi pressure, but few people are aware of that. So, moderators, you might want to lock this thread up before it goes viral or some other time wasting tyrade that gets way out of hand. Last edited by Bart B.; July 30, 2014 at 05:32 PM.

July 30, 2014, 06:29 PM	#5
steve4102 Senior Member Join Date: December 23, 2005 Location: Minnesota Posts: 2,955	The 7.62 and the 308 run at the same pressures, or close. Unclenick has posted on here a few times as to the differences or lack there of. This is a quote from one of his previous posts. Sorry no link. If you look through military ammo specs from before the mid-90's, you find military copper crusher numbers reported as "psi" at the same time a commercial maker would have reported them as "CUP" (I don't know why "CUP" gets caps while "psi" gets lower case, but that's how SAAMI publishes them). The SAAMI conformal piezo spec is 62,000 psi MAP (Maximum Average Pressure). I'll have to check, but I recall SAAMI MAP allows up to 4% deviation from average, so 64,480 psi would be the allowed extreme spread in SAAMI compliant ammo. The NATO allies use 415 MPa, which the CIP uses for both 7.62 NATO and .308 Winchester. That converts to 60,191 psi, but the CIP measuring system sets the sample point further forward in the case, so it sees a bit of the pressure drop at the bullet base that occurs when the bullet starts moving. That makes them tend to read about 2,000 psi lower than the SAAMI setup does in this pressure range. So the same ammo would measure closer to 62,000 psi in SAAMI equipment. You can also check out Commercial 308 and 7.62 ammo, here http://www.midwayusa.com/product/953...oint-boat-tail http://www.midwayusa.com/product/953...open-tip-match Both have identical velocities, if they differed in pressure by the claimed 12K then velocities would also be reduced. As this is a reloading forum, check out the 7.62 load data. Not the data that is rifle specific, data that is chamber specific. Guess what you can't, as there is no data that I am aware of that is specific to the 7.62 Chamber. With thousand/millions of rifles chambered in 7.62 if they required their own data, you would think one or two of the powder manufacturers would cater to the 7.62 round, but they do not. Why? Because it is a 308 Winchester. Now when using 7.62 Military Brass be aware that you may have to adjust your load due to reduced case capacity to stay within safe pressures for both the 308 and the 7.62. You can also check this out. http://how-i-did-it.org/762vs308/chamber.html Last edited by steve4102; July 30, 2014 at 06:35 PM.

August 4, 2014, 07:06 AM	#7
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	Milspec ammo must also meet port pressure requirements. For years folks knew that 30-06 was 30-06, but that the Garand shouldn't be fed just any random hunting ammunition. Same for the M1A/M14. The DI system for the AR family is a little more forgiving, but still it is a consideration. And completely aside from the calculated pressure of a particular powder charge you run into Secondary Pressure Spikes with some primer/powder combinations which can give you the same symptoms as an overpressure event. This seems to happen more with ball powder than extruded. As far as pressures and dimensions go, I interchange 5.56/223 and 7.62/308 all the time and think nothing of it. Do a standard load workup to ensure it is safe in your rifle than load up a lot of ammo and have fun. Jimro __________________ Machine guns are awesome until you have to carry one.

July 30, 2014, 05:03 PM	#2
chiefr Senior Member Join Date: January 27, 2010 Location: AR Posts: 1,401	I see no sense in continuing the argument, but I reload ammo for AR rifles identical as mil spec as possible. Conversely, in my .223 bolt gun, I load to near max and with heavier bullets. Same with 308. If the gun is mil, I load to mil specs. One of the advantages of reloading is you can have it your way.

July 30, 2014, 07:09 PM	#6
Kimio Senior Member Join Date: January 2, 2011 Location: Arizona Posts: 1,171	Thank you for the replies and the resources. This helps clear a lot of things up.