Knowledgebase: Dies
223 Remington and 5.56 X 45 NATO
Posted by Steph @ Lee Precision on 06 April 2012 12:54 PM

These cartridges are externally the same specifications. Case wall thickness is different and will affect the allowable charge size. You will find a difference in rifle chamber dimensions, but either cartridge should function in either chamber.

The cartridge specifications just above the rim are .3679/.3759. Chamber specifications at the same point are .3769/.3789. So as you can see, even at worse case, cartridge large and chamber small, they should still fit together.

Another more critical dimension is head space, the distance from the head of the cartridge, or bolt face, to a point on the cartridge shoulder defined by a diameter, or a point in the chamber defined by the same diameter. At worse case there is a .003” difference between cartridge and chamber, the chamber being the longer. If the resizing die is not set up and used properly, the headspace of the cartridge can end up being too long. This is the most common problem with cases not chambering. Unfortunately most people can not measure head space.

Our die produces a case diameter small enough to chamber in any rifle. If your rifle has a short chamber, or you didn’t size the case properly, it may not chamber.

To set the resizing die, screw it in until it touches the shell holder. Drop the ram and give it an additional ¼ turn. When sizing the case use LEE Resizing Lubricant. It makes a difference. Make sure the shell holder touches the die at the top of the stroke. You can check this by placing a piece of paper on top of the shell holder. You should not be able to pull the paper out when the ram is topped out. If the cartridge still will not chamber, return the die to LEE Precision and we will shorten the die to minimum head space.