If you build a rifle, and the lower is marked 5.56,this is no indication that the chamber is not a .223 or a wylde.I have a shaw barrel with no markings. This should be considered when selecting ammo.I always buy .223 because Im not sure what chamber I have.And what about ammo from say Sportmans guide, or Cheaper than dirt?  The print says .223, the ammo box says 5.56? This is a safety concern for me as a new guy.

How the lower is marked means very little, I have Colts marked .223 but the BBLs are chambered for 5.56.  A replacement BBL could be Wilde, .223 or 5.56 chambered. If it is a Shaw BBL was it a replacement? Or, did it come from the factory as such. If it did, a call to the factory would be your best bet. If you do not know, I would err on the side of caution and shoot .223 only. A call to shaw might also be in your best interest as they could also tell you what they usually chamber their BBLs in.