I picked up a bunch of empty 5.56 cases at the firing range this weekend and after a primary sorting I managed to get 24 that were in usable condition. I cleaned them up and deprimed them.

After resizing and priming I started to develop a load for 5.56 brass so for investigative purpose I put 26 grains of Varget in the case. To my surprise the cases filled to the same height as a standard .223. I tried this with most of the cases, but did not load any yet.

Is this a normal situation, where 5.56 brass has a close case capacity to .223? Is it still necessary to reduce the load by atleast 10% if case capacity is similar?

Another question: Could a person use magnum primers where there are reduced loads for 5.56 brass?

I'm surprised at your surprise! The 5.56 X 45 M-16/AR-15 round is the .223 in military jargon. Some may say there are differences but they are so small as to be insignificant. Yes, the case capacity should be almost identical with the possible exception of military cases not holding as much as a commercial case due to the possibly, thicker brass made for full automatic weapons.
For reduced loads, the magnum primer would be safe but not consistant with a reduced load. If you use Remington #7 1/2 primers, you can't go wrong. They were designed specifically for the .223 REM/5.56 M/M.

There is so much emphasis on 5.56 brass having less case capacity and the fact that I have never loaded 5.56 brass was the reason for my surprise. I know that the 5.56 and .223 are almost 1 and the same. I thought that there would be a noticeable difference to make me want to reduce my loads since, again, I have never loaded 5.56 brass and just heard and read a person should reduce powder by 8-12%.

Well, it sounds like you got past priming. Otherwise you have to watch out for crimped primer pockets in mil brass.
- mike

quote:

Originally posted by Masterifleman: Some may say there are differences but they are so small as to be insignificant.

Not so. I had a Savage 24V that would shoot commercial .223 but not military 5.56x45mm; the firing pin didn't quite reach the anvil. Something different in the chambering. I've heard it might be related to the shoulder position or something similar.

5.56mm chambers are slightly larger than .223Rem chambers to allow for rapid and reliable chambering and extraction in full-auto weapons. The leade in a 5.56mm is also a tad longer than in a .223Rem.

quote:

Originally posted by TomP:

Not so. I had a Savage 24V that would shoot commercial .223 but not military 5.56x45mm; the firing pin didn't quite reach the anvil. Something different in the chambering. I've heard it might be related to the shoulder position or something similar.

And I still say, after repriming hundreds of 5.56 mil carts. with commercial primers, with 100% success rate, that the Mil primers are too hard for many commercial rifles.
Note that I did not resize or otherwise change the case dimentions.
The danger if any is hot mil loads in tight commercial chambers MAY be too much (of a good thing)