I read through the FAQ above about the differences between 5.56mm and .223 Rem and I basically got the impression that it's not a good idea to use 5.56x45mm in a rifle chambered for .223 Remington... but what about the other way around? My Bushmaster has "CAL. 223-5.56MM" marked on it so I'm guessing it's okay to shoot either.

Also, I'm hoping Winchester .223 Remington is decent for plinking. Walmart has it for $7.50/40 rounds and while I know it's not the cheapest overall, it is the cheapest when you need it right now and can't wait for the next gunshow or ordering it online.

[b]YES![/b]


now go out there and have fun (but be safe about it!) [:)]

Go for it.  5.56 is dimensionally the same as .223, but operates at higher pressures.  .223 out of a 5.56 is good, 5.56 out of a .223 might cause a few problems.

The cartridge casings for both calibers have basically the same length and exterior dimensions.

The 5.56 round, loaded to Military Specification, typically has higher velocity and chamber pressure than the .223 Rem.

The 5.56 cartridge case may have thicker walls, and a thicker head, for extra strength. This better contains the higher chamber pressure. However, a thicker case reduces powder capacity, which is of concern to the reloader.

The 5.56mm and .223 Rem chambers are nearly identical. The difference is in the "Leade". Leade is defined as the portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. It is also more commonly known as the throat. Leade in a .223 Rem chamber is usually .085". In a 5.56mm chamber the leade is typically .162", or almost twice as much as in the 223 Rem chamber.

You can fire .223 Rem cartridges in 5.56mm chambers with this longer leade, but you will generally have a slight loss in accuracy and velocity over firing the .223 round in the chamber with the shorter leade it was designed for.

Problems may occur when firing the higher-pressure 5.56mm cartridge in a .223 chamber with its much shorter leade. It is generally known that shortening the leade can dramatically increase chamber pressure. In some cases, this higher pressure could result in primer pocket gas leaks, blown cartridge case heads and gun functioning issues.

The 5.56mm military cartridge fired in a .223 Rem chamber is considered by SAAMI (Small Arm and Ammunition Manufacturers Institute) to be an unsafe ammunition combination.

Look at the top of your barrel in front of the handguard. My heavy barrel has stamped BMP 5.56 NATO 1/9 H BAR. The lower is stamped CAL. 223 - 5.56 MM. It's my understanding that Bushmasters are made to Mil. Specs. so the chamber is sized for 5.56 ammo.

About 9 to 10 thousand rounds should be ok.