What is the difference? How can you tell what your barrel is set up for?

5.56 has a 5.56mm neck, .223 has a 5.54mm neck. .223 has a tighter chamber too, and a shorter lead.

Your barrel should be marked. Who made it?

For information on the differences between the two:

Ammo Oracle

To find out what the chamber in the barrell is, I'd contact the manufacturer and specifically ask.  Most are stamped on the barrell.  From what I've read most of the big AR makers chamber in 5.56, but I don't  have all the facts and I don't want to name names & steer you wrong.

CD

I just bought a barrel from Del-Ton. I have not recieved it yet (bought it today Friday). The barrel is as follows....Amalite 20" heavy barrel post ban chrome lined 1/9 twist. If you go to Del-Tons site it is product number 1481 in their barrel selections under Armalite. Please tell me I will be alright.

Armalite's should have a 5.56 chamber, although I hear that some are really .223. Olympic uses .223 chambers and I have shot lots of 5.56 through them. You should have no problems.

From Oly's site, about their barrels:  

"Our standard barrel button rifled...  is chambered in 5.56 NATO...  have 6 lands and 6 grooves in the bore."  (Standard barrels are stamped 5.56, and if stainless, the letters SS)

"SUM (Stainless Ultramatch) barrels are all chambered with minimum SAAMI 223 Remington reamers. These barrels can also be identified by looking down the bore. All of our broach-cut or Ultramatch barrels have only 4 lands and 4 grooves."   (SUM barrels are stamped SUM.  These are Oly's target bull barrels)

There is a whole page about Oly's barrels, I suggest reading it.

From Armalite's site, Library> Tech Notes > #45

"NATO and SAAMI cartridges can normally be used interchangeably with no problem." (My italics)

"Millions of rounds of NATO ammunition have been fired safely in Eagle Arms' and ArmaLite’s SAAMI chambers over the past 15 years." (Again, my italics)

"ArmaLite has adopted a practice of using a special, modified SAAMI chambers in its stainless steel match barrels.  This chamber is better for match use than the NATO chamber, but fires the NATO ammunition perfectly.  We use the NATO chamber in all moly (phosphated) and chrome-lined barrels."

Armalite has a great wealth of AR-15/M16 knowledge in their Library of Tech Notes.  Some good reading and highly recommended.

It never hurts to go to their sites and get the info you need.

Guys that buy a target bull barrel for their AR-15's generally do not plan on firing surplus, Wolf, and other such ammo.  The SAAMI chamber is well suited for their purpose.  

Quoted: Oly has used 5.56 chambers on their regular barrels for many years.  Only their SUM (Stainless UltraMatch) target barrels have .223 chambers.

Even the ones marked "556" are almost always .223. Ask Tweak.