One issue I did have 'the other way around' was in shooting commercial Winchester 45 grain 'Varmint' loads in my semiauto Bushmaster M17S Bullpup - itt would fire several times - slam-firing. All other .223Rem commercial ammo and all surplus 5.56 I tried worked fine. I suspect wery soft primers in the Varmint loads.
Accuracy usually loosens when shooting .223 in an AR. My AR has a Wylde chamber so it shoots .223 a lot more accurately than a standard 5.56 chamber. http://thefiringline.com/forums/showthread.php?t=483719
I have been shooting 556 in my bolts rifles and Ar's never had a problem. Id imagine if you had a match grade chamber that was first off of the line and to the tightest tolerances and a 556 loaded a little warm round, and a very hot day, I could see pressure troubles. But if you got the last rifle off of the line, with a loose chamber because a chamber reamer just went through a couple thousand rifles before it and light load, you would never see an issue . Good topic and conversation .
That AR came with a Wylde chamber which got me curious. RRA and others use a Wylde chamber. Essentially it is a tightly dimensioned 223 chamber with a longer throat. IMO, the Wylde's popularity is it allows optimal seating of 80 grain bullets. As my rifle shoots under a MOA with either ammunition so I am not complaining.
I usually don't throw hearsay into a fact-based mix of statements on a subject like this, but I long ago heard (25+ years ago) that the early versions of the Ruger Mini-14, though listed as chambered for 223 Rem., were deliberately bored "on the loose side" to more easily accomodate the 5.56mm cartridge dimensions. This could contribute to the "less accurate out of the box" reputation of the Mini-14, but doing something like that for safer presssure peaks and cartridge mixing convenience would be characteristic of how Bill Ruger did things. Ruger has probably long since updated their Mini-14 chambering selections to reflect the milspec option. I haven't checked--no dog in that fight--just my 2 pennies. YMMV
That is interesting and would explain alot of my troubles I had with all of the mini 14's I owned in the past, I loved everything about the rifle except the inaccuracy.
"At an unspecified time in 2007 to 2008, Ruger added a heavier tapered barrel to the mini series. The heavier barrel had an overall larger diameter with the barrel visibly becoming thicker in the final inches as the barrel approaches the gas block from the muzzle. These changes combined with tighter tolerances result in greater potential accuracy.[5] The new mini-14 rifles are arguably capable of shooting under 2 MOA (Minute of angle) accuracy. The "target model" Mini-14 supposedly can shoot under 1 MOA." http://en.wikipedia.org/wiki/Ruger_Mini-14#Production_versions A heavier barrel redesigned for accuracy and tighter tolerances yielding 2 MOA as a result echoes your experiences..
