For a Carbine the A1s are Definately the Superior sight. Why?
1) Carbines are supposed to be light, you save that 1.6 oz with the A1 upper.
2) Carbines are supposed to be close range weapons. With a 50y/200M zero you get a nice flat zero to 220M and you can easily hit man sized targets to 300M.
3) KISS principle. You set and forget. There is no 'accidental' changes on the A1 sight system. BTW its also the most durable sight system available for the AR family. In real world combat, and in ISPC type gun-games there is NO time for changing the elevation wheel.
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#1 If .1 lb for the sight makes such a difference, why not just get a 16" skinny barrel and save another .5 lbs?
#2 I can have the same zero with A2 sights
#3 I agree. I have never encountered any accidental changes on A2's, though it is possible but not possible with A1 sights.
The .1 lb savings alone really isn't enough to make me want to not have A2 sights. Especially since I don't plan on getting sent to combat any time soon, so what do I care. Considering the M4 barrel weighs a full half pound more than a 16" skinny, I don't see what big deal the .1 lb makes.
In fact my flattop with M4 barrel, Phantom and carry handle weighs in at 3.4 lbs. With M4 handguards it's STILL as light as a government A2 upper, which isn't heavy at all.
As I said before unless you are counting half ounces for a superlight upper, it won't make much difference. For lightweight I say get an A1 upper with a 16" skinny barrel. That weighs in at 2.4 lbs. Get a colt shaved bottom carrier and take the heat shields out of a set of regular carbine handguards and you'll be all set. THAT's lightweight, but not M4 which I kind of thought the original question was going for.
So it just depends on how close to "M4" you want to stay, how durable you need your sights to be, how much weight is a factor, and if you may need to make any expedient sight corrections in the field (though highly unlikely).
For an "M4" the flattop is nice, because you can add on all sorts of neato gee-whiz optics when money allows. Otherwise A2's are plenty durable and are more likely to be at the range then in "combat". A1's are indeed the most durable and save .1 lb on weight over A2's, though they are pretty much set when you get your zero.
Just some interresting weight facts from Bushmaster's website:
XM177 Bbl. - 1.7 lbs.
M4VO Bbl. - 2.2 lbs.
20A2 Bbl. - 2.5
20" - Heavy Bbl. - 3.5
A1 Type - 0.7 lbs.
A2 Type - 0.8
Flat-top - 0.6
Flat-top - 1.2 (including weight of A3 Type Removable Carry Handle)