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The 16" barrel I use for work and has an Aimpoint but at qualifications at least once a year they make us qualify with iron sights with the Aimpoint turned off.
Wont use it often but I want a good quality one.
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I like the LMT sight for that scenario, Daniel Defense also has a nice A1.5, but the LMT sight is most likely what you are used to. It isn't a flip up, but I think most people with Aimpoints like them.
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My 20" heavy barrel I plan on mounting a good quality scope for hunting Coyotes, Bobcat etc.. May not actually need a rear sight on this one except for SHTF scenarios. If I do it I want quality. All I have ever shot is carry handle AR's.
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I like the KAC sights for that. I prefer the standard rear, as opposed to their smaller line, but it doesn't really matter for the rear. I strongly prefer their regular front sight, but they are pricey.
For the ones that I don't feel like spending that much on, I usually get a Matech off of the EE. I've been paying around $40, shipped. I don't know about front folders, other than the one I mentioned, as most of mine have fixed front sights.