Remington developed 223 Remington to meet the US military requirements at the time.
The original, pre-Colt Armalite AR15 chamber had a long throat. This was the chamber Remington originally used.
Right before Colt bought the rights to the AR15, Stoner and Armalite shortened the chamber throat. This was the original chamber Colt used before the XM16E1 and the M16A1.
The first large quantity military purchase of 5.56 ammunition from Remington was in 1964. Reports of overpressure issues started the same year.
An investigation into the overpressure problems found that the chambers were different between Remington and Colt. Colt never had the original long throat chamber as part of the design.
Remington changed to the short throat chamber that Colt had been using. Remington then reported that they could not reliably meet the military velocity requirement while keep[ing within pressure limits using Dupont powders, which owned Remington at the time.
In a search for a solution, the powder was changed to Winchester Ball Powder and Dupont did work up some newer IMR powders.
Remington used the Colt short throat chamber as the standard 223 Remington chamber for SAAMI specs.
Later the whole M16 rifle program went through some upgrades. there were some buffer changes, a forward assist added, a chromed chamber and later a chromed barrel bore, flash hider changes and a long throat barrel chamber that was patented by Colt.
Originally there were no industry methods for chrome lining 22 caliber rifle bores. The requirements are different from larger bores. Colt developed and patented a method to chrome line 22 rifle bores.
With the newer, long throat Colt chamber more powder in the cartridge was required to meet US military velocity requirements.
The pressure limits remained the same but cartridge had more powder that would increase the pressure if fired in the older, short throat chamber that had become the commercial standard.
In the 1970’s FN developed the belt fed Minimi machine gun at the request of the US military.
FN developed a special cartridge with a two core bullet for use in the Minimi. This was the SS109 cartridge.
The chamber limit was slightly higher than that of the M193 cartridge. Not hugely, but enough to push the case to its limits for maximum performance.
NATO standardized the 5.56 cartridge based on the FN SS109 cartridge.
Being a belt fed machine cartridge, it wasn’t originally designed for accuracy.
During the 1991 Gulf War the US found producing enough M855 ammunition that met accuracy requirements difficult and most US 5.56 ball ammo issued during the war was M193.
When military firearm pressure standards changed for copper crusher to transducer measurements, the pressure limits for M193 and M855 were made equal. The velocity requirements remained the same so, M855 may be loaded closer the pressure, “limit” than M193 is.
The US Air Force still uses M193 as they still have many older M16 carbines designed the cartridge that get little use other than standing guard.
Even with a faster rifling twist rate, the M16 carbine is less reliable with M855 than M193 ammo. This is one reason that lead to many updates to the M4 carbine. Other reasons include new Colt patents, Clinton changes to,“Mil Spec” requirements and lobbying to get the US Army to switch from the M16A2 to the M4 and get Colt back the rifle contract from FN.
M193 was originally developed by Remington in a chamber that was never used by Colt.
When this was discovered, Remington switched the 223 Remington to the original, short throat Colt chamber.
The upgraded M16A1 resulted in a change to a new, Colt designed long throat chamber.
Remington didn’t change the commercial 223 Remington chamber to match the Colt designed chamber and the long throat chamber requires more powder to meet velocity requirements,
The additional powder for the long throat chamber can cause over pressure in the short throat 223 Remington chamber.
For all practical purposes you do not have to sort between the two cases. After you work up your favorite load you can try separating cases to improve accuracy but it is not necessary for general loading and I have never bothered to separate them with no ill effect that I’m aware of. Sound advice for all beginning to reload is to buy a couple of reloading manuals from your LGS and read them from cover to cover. Only then will you have a solid idea what you want to accomplish and how to go about meeting your goals. You will use your manuals throughout your reloading process and they are an excellent resource to answer damn near any questions you run into. Take your time and have fun!
