1/2inch. One day 3" next....69gr SMK 27gr varget.......65gr SGK. 26gr varget.....does new federal AR15 primers make such difference over GM 205 match?

Welcome to the forum.

Those primers will be less sensitive to prevent slamfires from the floating firing pin in the AR, so it's a safety measure. The inexpensive Russian-made TulAmmo KVB556M is a good, consistent military rifle primer, too. The CCI #41 is CCI's entry into the mix, but I don't find them as good with velocity constency as the Federal and TulAmmo primers.

__________________
Nick

__________________________
Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Instructor
NRA Patron Member

"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley

Nick has you covered for the primers, personally I like Win primers. Never had an issue with them running away, FWIW.
As to the "accuracy" comment, Could be several things. The one that jumps out at me first is the powder/weather. Varget works very well in the 308 case, but struggles with temp changes in the 223 IME.

Try Reloader 15. With that twist, try 75-77 grain bullets.
+1 about Winchester primers.
Pete

__________________
"Only hunting and mountain climbing are sports. The rest are just games."