I have been told...very confusing, that in a AR15 "upper receiver" ...as well as AR15 barrels there are "M4" type, deep-cut/angle FEED RAMPS vs. non-M4 "type" or "A3" ramps[/B]? Then at Daniel Defense website...further confused as they call theirs "A4" feed ramps and say that that is the deeper cut/angle "type"? So "M4...A3...and now A4" which seems to sound like a "hybrid term" of the 1st TWO??? What's the deal with this... ...I thought you could just "change" the barrel on a 5.56 rifle to a .300AAC BO. barrel, since the bolt carrier & mags are the same...(especially, one that allow that easily, like the LaRue...?) Was completely unaware that you could have un-evenly "matched" feed-ramp issues and potentially have bought the wrong "type" barrel? Also, what about AR10 "type" of rifles in 7.62x51/.308 and switching calibers to .338 Federal, with just a NEW barrel in that caliber [/B]...could you could do that on a Rifle like LaRue PredatOBR, "quick change barrel / suitcase rifle", WITHOUT these "FEED-RAMP" matching issues, that 5.56 vs. .300AAC BO. suffers from... ... as, uppers & barrels, apparently use differing feedramps that MUST be "matched" barrel to upper?