CORRECT CHAMBER SIZE

[newguy]

recently Ibrought a couple of my AR,s to a gunsmith to the chamber size checked tosee if they were true 5.56 mm they were but my question is how did he determine the crrect size he was,nt there when I picked them up for me to ask how it was done.CANCHAMBER SIZE BE CHECKED BY USING A HESAD SPACE GUAGE if so please explain how. If not please explain how this is done and the tools needed to do it

[jmart]

No.

Chamber cast (cerrosafe) or use the guage that Ned Christiansen (www.m-guns.com) designed.

[DrBackJack]

Maybe he just looked at what was stamped on the barrel :D

[SWATcop556]

Very very very few rifles that state they are 5.56 actually are. Ned Christianson make a gauge that is very easy to use. His chamber reamer will tune a chambr to 5.56 specs. Even some of the LMT uppers I've seen have some metal shaved out so a true 5.56 spec chamber varies.

I would want clarification on how he checked the chamber.

[Thomas M-4]

No the difference is in the throat not the head space.
http://www.m-guns.com/tools.php
This is the proper tool to check the throat if you look at the pics it shows you what it is measuring.

[Ned Christiansen]

Oddly, I have yet to run across more than maybe one AR-15 where headspace was not right. All the other easy stuff they miss sometimes and yet this is so rarely wrong.

Freebore and throat, that's another story. A friend recently did some pressure barrel testing and found this:

"What does all this mean? In short, you can safely fire all 5.56 AND 223 ammunition in a gun properly chambered for 5.56. You MUST NOT fire 5.56 ammunition in a 223 rifle. As case in point, I fired XM193 5.56 ammunition in a 223 test barrel with average pressures (conformal transducer) of 72,550 psi, and peak pressure registered at 76,250 psi. Continued shooting of 5.56 ammunition in guns not chambered for 5.56 will show many warning signs of over-pressure, such as flattening of primers, smearing of the head stamp, dropped primers, blown primers and pre-mature wear on extractors and bolts."

Another friend had a batch of department carbines with .223 chambers that had been shelved because of chronic popped primers. After they had been covered by enough other stuff in the armory and basically forgotten, he figured he could get away with a little experimentation (sshhhh....). He measured each chamber forward of the shoulder using the Stoney Point tool. He grouped and chrono'd them with XM193 and American Eagle, reamed them with my reamer, then re-tested.

Long story short, the one that measured the shortest in terms of chamber throat gave him velocities of up to 3400 FPS.

I said 3400 FPS. These were 16" carbines. Three-thousand, four hundred feet per second. Who the hell needs a .22-250?!

The one that actually reached 3400 FPS..... popped it's primer, big surprise!

He reamed the lot with my reamer and velocities came back down to where they should be in a 16" carbine-- in the 3000's, some in the 3100's (that XM193 is not wimpy stuff). And no more popped primers.

Bear in mind that my reamer makes a chamber that is actually a tad longer than 5.56 NATO. Is it an accuracy killer? Not in my testing-- I have actually found XM193 to group tighter with this chamber. This chamber is actually not so unlike what you'll find in Camp Perry guns optimized for the heavier bullets.

Hope that helps.......

[Molon]

"What does all this mean? In short, you can safely fire all 5.56 AND 223 ammunition in a gun properly chambered for 5.56. You MUST NOT fire 5.56 ammunition in a 223 rifle. As case in point, I fired XM193 5.56 ammunition in a 223 test barrel with average pressures (conformal transducer) of 72,550 psi, and peak pressure registered at 76,250 psi. Continued shooting of 5.56 ammunition in guns not chambered for 5.56 will show many warning signs of over-pressure, such as flattening of primers, smearing of the head stamp, dropped primers, blown primers and pre-mature wear on extractors and bolts."






Outstanding information Sir! Thank-you for posting that. With the help of two of your pictures, I've put together some graphics to aid in visualizing what happens when a 5.56mm round is fired in a SAAMI .223 Remington chamber.


A SAAMI spec .223 Remington chamber will have a shorter leade with a sharper angle to the leade and a shorter amount of effective freebore than a 5.56mm NATO chamber. The freebore itself will also be narrower in the .223 Remington chamber.


http://www.box.net/shared/static/oobedcsfkc.jpg


(The following is just a generalization to demonstrate the concept. DO NOT hold me to the exact numbers as they are not correct and they ignore the difference due to the different methods used to measure chamber pressure.)

Consider the left graph pictured below; M193 fired in a 5.56mm chamber. The pressure is within the MAP limit. Now, take the exact same round, (same powder, same charge of powder) and fire it from a .223 Remington chamber; pictured in the right graph below.

Because the .223 Remington chamber has a shorter and sharper angled leade compared to the 5.56mm chamber as well as a shorter effective free-bore, the bullet engages the rifling sooner in the .223 chamber than it would have in a 5.56mm chamber. This causes the pressure to rise faster, peak sooner and reach a higher (and per SAAMI, unsafe) level than it would have if the round had been fired from a 5.56mm chamber.


http://www.box.net/shared/static/rkuh8gzlj2.jpg