Interested readers-
Here is the interpretation from John Amidon:
"You are absolutely correct, the AR-15 type rifle has been made in far sufficient numbers to not be considered a prototype, and yes, the factory gun appendix allows for replacing the barrel, and no, there is nothing ruling about changing to a different caliber.
The rifle rules appendix for Limited and Tactical has no reference to cartridge other than none, the rules themselves state: 5.1.2 There is no minimum caliber for rifles used in USPSA matches.
The only mention of caliber is in the Multigun rules, and that is under the heavy metal divisions, it must be a minimum of .308.
The appendix containing the word prototype covers the rifle, there is no reference to ammo.
The user was able to obtain major scores, because his ammo made major over the chronograph.
Hope this helps you clarify your questions, the rifle used, was legal."
That's the interpretaion from USPSA.
scooterdog