I got a good read on the twist and as I suspected it is the old standard 16. Sierra is the only source of .223 bullets these days and only in 40 and 45 grain (when they are available). I will exhaust my tests on these before moving to .224 bullets and if the 40 .223 turns out to shoot better than the 45 .223 may not try heavier .224s. All that will be in the future anyway as the rifle is now at my gunsmith's to have a 32-20 BRC barrel fitted and my next tests will be with the 32-20.