I also have an old Colt SP-1 in the 57K serial number range. I have owned this rifle for quite a few years and never fired it. On the side it says .223 and the barrel is only marked C MP Chrome bore. I was always told the at the SP-1s were really .556 no matter what it said on the side of the rifle. I now have the urge to fire it but not sure what caliber ammo to buy. My local gun shop has 400 round cases of .556 for less than $200 which seems ok in today's market. I just want to make sure i am getting the correct fodder for the rifle. Thoughts?

Dave