It'd be nice to have a 9mm version of my .45 for cheaper shooting.  Just ordered a 509MRD, but I would rather have a DA/SA.

How much does RMR slide milling cost?

Yes it would be cool to have a 9mm version of the FNX tactical but unfortunately, the fnx-9 never sold well enough and now DA/SA is considered to be a mostly obsolete system.

The 509 and sig p320 are what is considered the most up to date pistol platforms now with p320 having a slight commercial edge in the sense that it is a chassis system.

Slide milling generally costs anywhere between $150 and $250 with various companies having a 1 to 10 week turn around time. The FNX-9 being a less popular pistol may make finding some to do the slide milling a bit more challenging.

^^ I agree. It would be nice for a da/sa option bu striker guns are what people are wanting and what is selling. My 509T is an amazing pistol and the trigger pull is a dream, right out of the box. I prefer it to any other striker fired pistol but it comes at a cost.

FNX/FNP pistols have the advantage of all wear parts being unserialized. Sort of the opposite of the P320 chassis system, but if you break a frame rail it is replaceable without replacing the frame.

I completely agree with you and see that as a major advantage but we are in the severe minority with that opinion. Most guys do not see it that way and would much rather have the ability to drop a chassis/FCU into a new frame of a different size, shape or color.

The argument that glock has proved that frame rails almost never wear out or break is a rather tough one to argue against.  

It's just a different way of doing things, and like any tradeoff has advantages and disadvantages. I was actually going to say that one of the things known to break on high mileage Glocks is the rear frame tabs, in addition to the recoil spring hole on G4 guns and slide rails; but that's usually at pretty extreme round counts so unlikely to happen to most people, and if you can afford to shoot it that much you can afford another gun. Glocks are cheap.

I would love an FNX-9T though, I love my FNX-45T and it's a blast to shoot suppressed but it's pretty expensive to do nowadays which sucks, so a 9mm would help with that. The FNX-45/T seems to be the only handgun FN has kept in production for very long aside from the Hi-Power, and 9mms are more popular than .45s so it seems like a no-brainer.

In the past couple decades they've had the BDA, BDM, Forty-Nine, FNP-9/40, FNX-9/40, FNS, 503, and 509 small frame pistols and none of them lasted very long. One of the main knocks on their pistols is that people are afraid to buy them because they're afraid they'll be discontinued in 3 years, even if the pistol itself is good. Meanwhile the FNP-45 came out in 2007, had a minor revision to become the FNX (the FNX small frames were essentially adaptations of the FNP-45) in 2013 (increased mag length, slide travel, and a change to the locking block geometry), and is still in production today. Glock hasn't even fully rolled out two newer generations of handgun in that time.

Don’t have a lot of new thoughts to add aside from I’d love a FNX-9T as well.

Really like my FNX-45T and I have the full size and compact 509 tacticals.  While the 509 is great, I do have a slight prefer to traditional DA/SA handguns.

I asked the same question. So here are a few things I figured out.

I have a FNP45 and a FNX9 and have been happy with them both, though neither was  threaded. A couple of weeks ago I went about fixing that with barrels from Midwest.

Fitted a FNX45 barrel for the 45 and got a FNS9L barrel for the FNX and threaded it.

Next was to mill the slide for an optic. I dont have any pistols with Optics so I decided to do it for this one. Did not bother on the 45.

The FNX9 slide is just too narrow on top for traditional RMR screw spacing. I was adding a Holosun 509, and it comes with its own plate. I milled it rather shallow, so no cowitness, and added new holes to the mounting plate to fit the narrower spacing on the pistol since the standard spacing is basically centered on the edge of the slide on the top.

Cant go too much deeper or you get into the firing pin channel, and on the right side of the slide there is a large void underneath. My mounting screw actually went all the way through the slide on the right side due to that void.  I also had to shorten the OEM rear sight to fit the optic.

All in all though, I like it. Put a few mags through it over the weekend and it is not reliable with the can yet. I think it still needs break in, I hadnt shot the gun that much before all this work.

Looks good!

I started out with the FNX45 tactical and liked it.  Then went looking for the FNX9 tactical, which doesn't exist.  Got the FN509 tactical, which didn't really do it for me.  Even with the apex trigger, its not that great.
Then scored a deal on a basically new FNX9 locally.  Ordered a threaded barrel for it from EFK, and thats as far as I made it.  I'm still lacking the optics cut and the tall sights.  The EFK barrel is a little longer than I would have expected, but doesn't look completely retarded.  I could always thread it back a little further if I wanted to.