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Can anyone shed any insight as to their......
Preferred twist rate. 1/7, 1/7.7, 1/8 (Cut vs button rifled)
Preferred adjustments on sights (I will be buying it with a carry handle so this is still relevant) 1/2-1/2, 1/2-1/4, or 1/4,1/4
Preferred front sight post width and size or rear aperture. (what issues would lead you to choose one aperture size over another and what are the benefits / disadvantages of going bigger or smaller?
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When I choose to build a National Match AR (or M14 for that matter) I pick Krieger first. Most barrels will shoot, but I've seen a higher percentage of Kriegers shoot well and for a longer service life than any others. I've also noted that they tend to be on the money for land/groove diameter - out of say 10 barrels, 7 or more will hold multiple easily sub minute 10-shot groups at 300 yards in a fixture. In a pinch I'll shoot WOA/Wilsons, Saterns, Bartleins, etc. without fuss, but I won't go above 3,500 rounds on any stainless match AR barrel. I'd gladly toss it as soon as I see noticeable velocity loss or several uncalled flyers at the 300RF with a tight knot in the middle.
My preference is Krieger with 1/7.7 twist rate, AMU chamber, headspaced to 1.436 with a Colt or BCM bolt and a BAT Machine extension (basically a .223 Wylde with a really long throat set to jump 75gr. VLDs about 0.035" over 24.3gr. or so of H4895 out of a LC case), a fully bored out flash suppressor, Surefire shims used to time it with minimal torque and a bit of red loc-tite. Too much flash suppressor torque can actually constrict the bore. I prefer a smaller gas port of around 0.090". Most military guns are 0.096", but 0.092" will work fine as long as you clean it and keep it lubed well. Mk262 likes this chamber too, so you're not losing anything at the short line.
For sights, 1/4 elevation is MANDATORY in my experience, 1/4 windage fine as long as you don't hesitate to move it 1/2 minute each time, 1/2 windage I actually feel is better as long as you can get a PERFECT no-wind zero. For irons WOA guide pins are the only way to go.
For irons I prefer a 0.040" WOA machined NM taper front sight, and a 0.040" rear short hood - this is all using an M16A1 or adjustable stock for a really close eye relief of 1-1.5". I want 2.5-3 minutes up from bottom for a 200SF zero to allow for flexibility in picking the hold I settle at. With this setup my preferred holds are 6 o'clock 200SF/200RF, 6 o'clock 300RF early in the season and center later in the season, and tight reverse flat at the 600SF. For the vast majority of shooters I suggest the same sight post but 0.052" diameter - not so wide you can't find lateral center, but not narrow enough to preclude those without perfect vision from shooting. I don't suggest more narrow than that for most. Any bigger and no-wind zeros appear to shift as you may not be able to track dead center with your eye, but elevation zeroes will be dead on (this is a generalization but seen across hundreds of shooters).
Hope this helps!
S/F