If I have a stripped FN A2 upper, could the Milspec irons be replaced with NM irons without any modifications such as machining?

If so, what about the rear irons in a detachable carry handle?

I am not sure on the NM irons, I think they will fit the A2 upper fine.

A2 upper/detachable carry handle sight parts are different, if I remember right.

Somebody with more experience on the subject will be along I'm sure.

if it's for competition use you'll want the machine work done.

Otherwise you'll have the side-to-side (actually rotational) play that a standard rear sight produces.

But yeah, they're basically the same.

A good place to get the rear sight pinned (to prevent rotational variation) is (of course) White Oak Precision

So it would probably in the end just be better idea to buy a NM upper?

a dedicated national match upper is always a good idea

Having a pinned rear sight is not necessary to shoot high master scores. There are many distinguished shooter that I know with out pinned rear sights.

Swap to the NM sights and shoot, if after shooting you feel that a non pinned sight is holding you back then get it pinned, by then you will probably need a new barrel anyways

this is true for most typical additional features, like freely-floated barrels, lead weights, optical sights, etc

Many national match sights are machined so thay they have much less play between the two wings of the carry handle.

The Armalight NM rear sight has a raised ledge opposite of the ball detent to ensure less side to side play.

Yes a standard NM rear will fit the A2 just fine.

For a carry handle you will need a nm sight designed for a carry handle as it has less elevation. Fitting a a2 carry handle sight will cause the threaded portion to extend past the bottotm of the detachable carry handle.

 Always a good idea.

John at WOP pioneered the pinned rear sight, he gets his parts from CLE. The only other pinned rear sights were First strike where Anna tried to one up John by adding a third pin, John is the only one still doing this. The two different types of play pinning reduces are twist and rock. Twist can be reduced by welding a tit to each lower corner of the front of the sight and then fitting it to the upper. Rock can be addressed by shimming the elevation wheel so it is tight in the upper. The bigger issue is will the sight track all the way to the left and then back to the right and up and down in measured clicks, there are a couple of dial gauges that measure this, Ray-Vin is a good one if you can find it.

Of course all of this is academic since fixed 4.5x scopes are the accepted norm now, for service rifle iron sights are dead. I know blasphemy but time and technology moves on.