What About the 10mm Upper?
Mark F. wants to know why anyone would want a 5.7x28 upper in the first place. Reader Heckathorn and others comment on Ruger detent dates. And why did the Colt show, and not place?
Let me start by asking, why not compare Olympics 10mm AR conversion instead of a 5.7x28 upper? When you ask, 'Is a $700 5.7x28 Upper a Good AR Buy?', I think, 'Is the 5.7x28 better at anything than a 223 or 22 Hornet?' My own answer is a resounding 'Nope!' If I want a small-caliber rifle bullet with lots of speed or penetration, the 223/5.56 seems more than adequate. If I need a smaller caliber than the 223, I can buy a complete 77-22 Hornet for less than the AR upper and feed it less expensive 22 Hornet ammo. The 5.7x28 may be of some value in a handgun, Ive never found myself in a situation to ponder that question and would speculate that I never will. If I were looking for a personal-defense rifle less powerful than the 223, I think the 10mm is a good start, as Ive determined that the 9mm makes a fine pocket-pistol caliber but a poor rifle.