have a 40c I bought in 2010. I have a 9mm barrel but I want to modify it. Where do I start? night sites, which one? apex? Which kit?

You'll most likely have the old SHB so you won't be able to fit an APEX kit until you get a new one which are very difficult to find now. You should get the H trigger bar too and new slide stop if you want a felt reset. S&W newer barrels are available also aftermarket too. For sights Trijicon HD's, Ameriglo or Truglo.

Why would I need a new barrel?

Are Xs sights any good?

There have been barrel changes. If you are happy with your barrel that's fine no need to change.

Some people like the XS Big Dots but personally I like the Trijicon HD's because the rear sight is blacked out in daylight and the front sight is highly visible. They also give a conventional 3 dot pattern at night.

I have a 2008 model 40c. I had to grind the frame to drop in a new sear block housing I picked up from GR tactical.
I have the apex dcaek kit and truglo tfx sites on mine.
Reliable and a great shooting edc gun for me.
Apex is the cat's *** for better triggers on these guns.
I also put the new slide lock release since gr has a minimum purchase policy... others say it has a more positive reset.
It's not a target gun, in a self defense scenario ...you will never notice that reset IMO.

The first question is: What do you use the gun for?
If it's home defense and/or carry I would say get some night sights (Tru-Glo TFO) andcall it a day. The stock trigger is MORE than suffient for those purposes.

If it's for competition use (which I doubt because it's a compact model) then that's a whole other issue, and one on which I am not qualified to comment.

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Stay protected my friends.

I disagree about the TFO.
It's an inferior design, the tfx are also night sights.
The apex dcaek kit make this a better, more consistent shooter.
Much cleaner break, less over travel, sits at +/- 5lbs.
Put lots of rounds down range, before and after...

OP is specifically asking about sites and trigger kit.