5.56mm chamber geometry

[kal]

I know that a proper 5.56mm chamber is said to be wider than a 223 chamber but in what way?

Is the entire 5.56mm chamber wider dimensionally or does the 5.56mm chamber have a more pronouced taper in order to force the casing to expand into a more tapered geometry to aid in extraction?

[bobbo]

I came across a thread Molon posted on this just recently, and the search came through for me for a change! :D

https://www.m4carbine.net/showthread.php?t=55149

[C4IGrant]

I came across a thread Molon posted on this just recently, and the search came through for me for a change! :D

https://www.m4carbine.net/showthread.php?t=55149

You know that the search function on the site works perfectly every time right?

https://www.m4carbine.net/showthread.php?t=58243


C4

[kal]

I came across a thread Molon posted on this just recently, and the search came through for me for a change! :D

https://www.m4carbine.net/showthread.php?t=55149

doesn't give me the info I'm looking for, which is the reason I made this thread.

Basically what I'm asking is, does the 5.56mm chamber have more of a taper to it than a 5.56mm/223 casing......

or.....

Is the 5.56mm chamber wider than the 5.56mm/223 casing from case head to shoulder?

Here's an extreme example....
http://farm5.static.flickr.com/4001/5167467142_7134fee7ea.jpg

[bobbo]

You know that the search function on the site works perfectly every time right?

https://www.m4carbine.net/showthread.php?t=58243


C4

I usually assume user error when I don't find what I'm looking for. :p

[bobbo]

Kal, if you look at the dimensions of chamber reamers that jmart linked to half way down that thread you'll see that the base diameters of .223 Rem. and 5.56 Nato chambers are very close to each other. With the 5.56 chambers being .002-.003 bigger.

[constructor]

The throat is larger in dia and the freebore is longer, the rest of the dimension including all case dimensions are the same.
I have a pdf with 8-10 chambers but the forum here limits the file size to 19.5kb and will not allow the upload.

[AMMOTECH]

See if this helps. This link is also tacked at the top of the page....

https://www.m4carbine.net/showthread.php?t=55149


http://img.photobucket.com/albums/v284/AMMOTECH/556cham.gif

http://img.photobucket.com/albums/v284/AMMOTECH/556natochamberversus223remingtonchamber02.jpg

Sorry I can't get them any larger.

.

[kal]

See if this helps. This link is also tacked at the top of the page....

https://www.m4carbine.net/showthread.php?t=55149



I already knew about the throat differences. I was more interested in the chamber dimensions. From what I understand, the chamber dimensions in relation to the casing dimensions is what determines reliable extraction.


Kal, if you look at the dimensions of chamber reamers that jmart linked to half way down that thread you'll see that the base diameters of .223 Rem. and 5.56 Nato chambers are very close to each other. With the 5.56 chambers being .002-.003 bigger.

Seems like some of the 5.56mm reamers are wider at the base than the shoulder compared to the 223 reamers. This seems to point out that 5.56mm chambers are a bit more tapered than the typical 223/5.56mm casing.

I realized the .002 inch is pretty much 1/20th of a millimeter. How the hell does that make a difference in terms of extraction reliability. :confused:

[spdldr]

It won't; unless the case becomes a tight or forced fit in the chamber. A human hair is approximately .004". That .002" is about 1/2 the thickness of a human hair.

For a number of reasons, extraction is one of the major weak points of the AR15-M4-M16 system. The cartridge case is a main culprit. If your carbine locks closed and easily hand extracts the unfired round, the chamber is not too tight.

A .223 vs. 5.56 chamber is another issue though.

Dave

Boonie Packer

[Clint]

For a number of reasons, extraction is one of the major weak points of the AR15-M4-M16 system. The cartridge case is a main culprit.

Dave

Boonie Packer

Can you elaborate on what the issues are?

The 556 case has small rim dimensions compared to other intermediate cartridges.

[spdldr]

The case is derived from the .222 Remington round which was never intended to function in a self loader. The rim is thin and of course, brass. Compare it to the Russian rounds in this area which were designed from the get go for autos.

Also, there is very little taper. This makes designing a feed system simpler, so that is actually an advantage. More taper in a round as high pressure as the 5.56 can be a real problem though, as it can become like a tapered fastening pin if the brass is a little soft. Note how difficult it often is to remove the two pins securing the front sight castle on an M4. The taper jams them in place. IF the round had a more reasonable lower pressure, more taper would possibly be better.

The 7.62 NATO is well designed. Moderate pressure, moderate taper, and a reasonably thick rim with an adequate extractor groove. So we did this one well. If we had used the .30 Remington case as a base for the M16 round, and utilized a better extractor, we might not have had as many problems with extraction. Lube your bolt carrier group AND your ammo for best extraction in the M4.

Dave

Boonie Packer

[jumbopanda]

http://ar15barrels.com/data/223vs556.pdf

[Clint]

These are excellent points.

I agree, the thin, shallow rim, in combination with the ridiculously high 556 pressures and early opening carbine gas systems can make extraction troublesome.

A stuck case is a terrible thing.

I looked into the taper, and the results may surprise everyone.
I calculated the body taper per inch or TPI for a few relevant rounds.

308 win - .012"

556,6.8,7x46, and 30-06 all are .016-.018"

7.62x39 - .038"

So it looks as if lack of case taper may be off the table as a contributing factor to 556 extraction difficulties.

The 308 only had 75% of that and may be a factor in 762/AR10 reliability.

The x39 has double the taper of the other rounds.

I read this was because the case was designed with steel to begin with and accounts for steel's reduced springback vs brass.

Also, we eventually did use the .30 rem case... To create the 6.8 SPC.

[spdldr]

"So it looks as if lack of case taper may be off the table as a contributing factor to 556 extraction difficulties."

Clint is absolutely right. In fact, case taper usually makes things worse. All that is needed is enough to feed easily and be freely removed after beginning extraction. American design counts on the elasticity of the brass case for "springback" and ease of extraction. This means that the hardness of the brass must be carefully controlled.


If the round is high pressure (like the 5.56) it may actually spring the locking lugs back just a very tiny amount when it is fired. Then when the lugs recover, they jam the case forward. If the case is tapered and softer than it should be, it is then jammed into the chamber and difficult to extract. This was a major problem in the Vietnam era. (there were several others too) High pressure is your enemy!

Dave

Boonie Packer