The chambers are what is different
223 bolt gun, 223 service rifle, and 556 nato. I am reloading for a Olympic flattop 16" AR15, barrel stamp is 556. Does this mean I should be using the 556 data? What would be the difference in using data for the other two.
First, look at the barrel of each of your rifles. The chambering (specific cartridge) will be stamped into the barrel.
A 223 bolt gun is designed to shoot SAAMI spec 233, which can be called commercial ammo.
"223 Service Rifle" is an Oxymoron. 5.56 Service Rifle would make more sense. It is One (223 commercial chamber) or the Other Service Rifle (5.56 chamber). You notice that your AR barrel stamp is 5.56.
5.56 NATO is the same bole caliber, but in these guns the firing chamber is longer than commercial 223. Specifically, it is the throat or Leade, which has slightly longer specs. There are some good diagrams of the chamber dimensions of 5.56 versus 223 here:
https://www.google.com/search?q=saa...hULilQKHSTMCAoQ9QEISzAD#imgrc=TvaE6HgOMq2BvM:
You will find explanation of the difference searching here
http://www.accurateshooter.com/cartridge-guides/223rem/
5.56 ammo is loaded to higher pressures than commercial 223, which is why different data is given. Western Powders reloading guide data includes both, and, the Sierra (5th?) have a separate a article and data for 223 and AR.
A certain rifle may have different chambering characteristics than another rifle in the same cartrigde from the same maker. I have a Savage Axis that will handle longer COAL, as long as 2.3x". I have another Savage 10 that will not close the bolt on anything longer than 2.26". I choose to reload and use separate ammo in the Axis with Hornady 75 gr BTHP, loaded as long as will chamber and fit in the magazine. They wont chamber in my other 223 rifles. I found this length buy using a dummy sized cartridge, with a 55 gr FMJ just started enough to hold it in place, and then closed the bolt completely (with a little force) and seated the bullet to the lands. Eject that cartridge and measure to get the max COAL for that specific gun only. I use that to set up my seating die to just a bit less length.
For your purpose, if the barrel says 5.56, you can uses 5.56 or 223 ammo. 5.56 ammo is not supposed to be used in 223 bolt guns as higher pressures may result.
Barrel twist has to do with heavier bullets and will your rifle stabilize them? 1":9" is the best all around and can do everything from light bullets to 75 gr. There are bullet stability calculators available. You need to know your specific bullet length and some other variables.
loading 25.1gr BLC2 with CCI400 primers and 55gr FMJ-BT mixed brass
1. You should use CCI#41 (milspec) primers or CCI 450. Your AR likely has a floating firing pin that can touch the primer out of battery. The 41's and 450's have thicker cups, and other differences to prevent "slamfires". I stopped using CCI 400's when I notice some fired primers had leaked or penetrated. Now I am using Russian 556M primers iintended specifically for 5.56 service rifles.
2. I cringe whenever I hear about people using mixed brass for bottleneck rifle cartridge reloading. Sure you can do that with pistols. But there is a lot more variability in 223/5.56 than in pistol brass. For 7.62/308 it becomes even wider in variation and would be dangerous to load mixed brass. I encourage you to get uniform brass (for example LC any years). Your local range may be able to help you out for free, or you can buy 200 LC Swaged for $22 delivered here: https://www.brassbombers.com/223-556-Cleaned-Deprimed-Swaged-Lake-City_c48.htm
