Yesterday I went to the local Cabela's and purchased the Hornady OAL Gauge. This is the tool that allows you to insert the test case into the breach, and by using a flexible cable, move the bullet up next to the rifling in the chamber. In other articles I have read, this is the best place to set your OAL.

Now first, I have a new Colt LE6920 AR15, so the amount of wear is not an issue. After following the instructions of inserting the gauge into the gun, the measurements I got were eye opening: The measured OAL was 2.38 inches, and at that length the 55 grain FMJ-BT bullet was just inside the case!! I have tried it many times and get the same results.

So, has anyone else used this tool, and what were the results?

The chambers for 5.56 NATO and 223 Winchester have different dimensions. You'll find almost all AR15s with long-throated NATO chambers.

"Q. What is the difference between 5.56×45mm and .223 Remington ammo?

In the 1950's, the US military adopted the metric system of measurement and uses metric measurements to describe ammo. However, the US commercial ammo market typically used the English "caliber" measurements when describing ammo. "Caliber" is a shorthand way of saying "hundredths (or thousandths) of an inch." For example, a fifty caliber projectile is approximately fifty one-hundredths (.50) of an inch and a 357 caliber projectile is approximately three-hundred and fifty-seven thousandths (.357) of an inch. Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

The chambers for .223 and 5.56 weapons are not the same either. Though the AR15 design provides an extremely strong action, high pressure signs on the brass and primers, extraction failures and cycling problems may be seen when firing hot 5.56 ammo in .223-chambered rifles. Military M16s and AR15s from Colt, Bushmaster, FN, DPMS, and some others, have the M16-spec chamber and should have no trouble firing hot 5.56 ammunition.

Military M16s have slightly more headspace and have a longer throat area, compared to the SAAMI .223 chamber spec, which was originally designed for bolt-action rifles. Commercial SAAMI-specification .223 chambers have a much shorter throat or leade and less freebore than the military chamber. Shooting 5.56 Mil-Spec ammo in a SAAMI-specification chamber can increase pressure dramatically, up to an additional 15,000 psi or more.

The military chamber is often referred to as a "5.56 NATO" chamber, as that is what is usually stamped on military barrels. Some commercial AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which provides for increased accuracy but, in self-loading rifles, less cycling reliability, especially with hot-loaded military ammo. A few AR manufacturers use an in-between chamber spec, such as the Wylde chamber. Many mis-mark their barrels too, which further complicates things. You can generally tell what sort of chamber you are dealing with by the markings, if any, on the barrel, but always check with the manufacturer to be sure."

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i agree with tigger tate. the hard part about stretching the 223 out so far is fitting them into the magazine.
you can try heavier bulletts if you have the twist rate to support going heavier.
if you want an accurat load, try aa2230 with the right bulletts for the twist and start with a oal out of a reloading manual and then start increasing the lenght by 10 thousands on an inch

Gary
At the risk of agreeing with one of our more vocal posters, put your OAL thingy back in the box and chalk your purchase up to "education" on limited usage tooling.

Once you get away from factory chambers, and into custom cut ones, THAN (and only than) should you pull it back out.