Originally Posted by
Circle_10
You're probably thinking of the later production model 723s. They used the "C7" style upper - basically an A1 with a brass deflector. Early production 723s used a 14.5 inch "pencil" barrel with a 1/7 twist, later 723s had the "M4" profile (although it wasn't called that at the time)
If I remember correctly the later 723s that used the stepped barrel profile, as well as the 727s (carbine with an A2 upper) were both basically using the barrel profile originally devised for the model 720 (which would later go on to form the basis of the XM4 program).
The USAF rebarreled a lot of their carbines and rifles with M4 profile barrels as well (and also with 14.5" pencil 1/7 barrels too) these were often designated the GUU-5/P. The USAF preferentially used rifles and carbines without forward assists for many years and basically just kept upgrading them with new parts as needed. There are a lot of photos out there of USAF carbines with slickside uppers and M4 barrels.
If you go back to the previous page you can see my own take on a GUU-5/P build.