Re: Bushing size for 223 Lapua and 77 grain SMK
First off, I don't know how successful you'll be shooting neck-sized ammo out of an AR. I suppose a lot will depend on the dimensions of your chamber. The general rule of thumb for a factory, 'no-turn' chamber is to load a round with your standard dies, then mike the neck in 2-3 spots. Take your average reading, and subtract .002-.003". That's what Hornady recommends, and Redding <span style="font-style: italic">used to recommend</span>, anyway.
Redding has recently changed their recommendation to .001" under loaded neck diameter, but they specify that this is for turned (or very consistent) necks used in benchrest shooting.
http://www.redding-reloading.com/techlinepages/bushingselection.htm
I think that you'll need more neck tension in an AR, unless you're single-loading, anyway. I think I'd buy bushings .002, .003, and .004" under loaded neck diameter, and see which one yielded the best accuracy and most consistent numbers over my chronograph. You may well find that you'll need a different bushing with a different lot of brass, anyway. Also, be sure to check for bullet setback after chambering.
Suggest you read <span style="font-style: italic">this</span> thread:
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1422166&gonew=1#UNREAD