Is there such a thing as a good reasonably priced rear sight,
fold down or fixed, to use on a flat top carbine to line up with
the A2 front sight? Most seem to be made of polymer and
quality is iffy it seems. Recommendations? Thanks.

I went a Yankee Hill Machine rear sight. Depending on the model they range from about $100 on up. Solid construction and feel. Here is the link to the one I got.

Flip Rear Sight

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YHM makes decant stuff. I had one of their LoPro flip-up rear sights on one of my past builds. No complaints, other that the little 'mouse trap' type latch that held the sight down. You might want to check out the MagPul Pro rear flip-up sight. All metal construction and the usual quality that you'd expect from MagPul products.

MBUS(R) Pro Sight – Rear

Or, another great choice would be one of the options from Troy Industries. Excellent quality and construction.

Rear Folding Sight -BLK | Troy Industries

If you want a fixed sight, this is a good option. Kinda pricey new, but can be found used for less... Probably a lot less now as these seem to have fallen out of style.

LEWIS MACHINE & TOOL AR-15 TACTICAL ADJUSTABLE REAR SIGHT ASSEMBLY | Brownells

For $50 shipped, Standard Manufacturing will send you a carry handle rear sight made in USA. I got one when I converted my 15-22 to M4 spec. I can't tell it from the Colt one on my 6920. At that price it's a steal.

Or go to Bravo Company and see what they have.

Check with Brownell's. They have about 5 pages of AR sight systems. Now, a word: you want to be cost effective, not cheap. This is particularly true with folding sights.

With that in mind, the Troy Industries sights are definitely worth the money. I'm particularly fond of the the Fixed Battle Sight ($70), but your needs may be different. In this particular case, the "fixed" means it doesn't fold. It is windage adjustable, the 2 different diameter sight apertures are the same height, so changing doesn't affect point of impact.

My Sport II came with a Magpul M-Bus folding rear sight. Later, I bought a no-name detachable A2 carry handle/rear sight that is perfectly matched to the front sight.

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Registration? NEVER!

High Quality Polymer

Magpul Industries MBUS Rear Flip-Up Back Up Sight Gen 2 | Up to 21% Off 4.8 Star Rating Free Shipping over $49!

Magpul's polymer rear sights are excellent quality and I have placed highly in matches with them. Ive tossed and beaten my rifle and they hold zero just fine.

Metal Rear Sights - All of these are excellent quality and Id put any of them on my rifles

LEWIS MACHINE & TOOL AR-15 TACTICAL ADJUSTABLE REAR SIGHT ASSEMBLY | Brownells

TROY INDUSTRIES, INC. AR-15 REAR SIGHT | Brownells

Folding Micro Rear Sight, 200-600 Meter Adjustable - Knight's Armament

A1.5 Fixed Rear Sight | Daniel Defense

I picked up a SW MP 5.56 AR in some trading with the A2 front sight and a scope. I don't need a second scoped AR so I've ordered a DD fixed rear sight. Should be here Monday.

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Isaiah 41:10

The polymer Magpul rear fold downs included on the S&W Sport rifles are not iffy quality as @AManWearingAHat said. They are a quality folding sight that will suit pretty much any use you have for it AND it will hold up. No need to change IMO.

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Still carrying my S&W 642

The UTG folding rear sight, available on Ebay for about $27, is machined aluminum and a good unit. I have them on 2 rifles and they have been completely satisfactory.

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Pisgah

Added this to my 2nd Generation Sport.

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NRA member since 1974