Blaine I appreciate you going to the mat for Interloper, I picked it at 4 and Sure Step at 6. Caltrops are so much fun, especially when dripping with poison... it's such a nice antidote to those smug melee enemies
edit: I'm up to level 6 on this build so far:
https://underrail.info/build/?HgMICgMMAwfCoGQAACgAwqBfNyYZU1NLKVMAAAAAQwAAJjs5HWE3RxZJT07ChQI-U0Hfvw
I may swap when I take Expertise vs Grenadier, it really depends on how throw-y my guy is.
I'd swap Expertise with Blitz in your feat order. Blitz is helpful and versatile, but by level 10, Expertise will be granting you +10 damage per round—just about the perfect time to grab it.
You probably don't want or need Ambush. For one thing, ideal face-spraying range is with the muzzle of your SMG practically up the enemies' noses, which makes it difficult to achieve the correct lighting gradient: either you'll all be lit, or all in the darkness. For another, Expertise adds 0 damage to crits—it functions only on non-crits. Crits are still more damaging than Expertise at the end of the day, though, and there will certainly be Ambush-able situations, so it's not useless. However, you haven't even planned Ambush until level 22, at which point you won't need it.
The power of Spec Ops SMG is such that, on Hard and below, criticals are unnecessary when you're bolstered by Expertise. This allows you to select great feats such as Blindsiding, which is more helpful to Spec Ops/Expertise SMG than many other builds, for very obvious reasons.
Gun Nut is... actually, probably a good choice, to help offset the W2C nerf if nothing else. It increases average damage, sans Expertise of course (since that's added on top), by 7.5%. I took Power Management on my Spec Ops SMG guy, though, which I didn't actually need since everything died before I could even turn on my shield emitter, so you're already on a better footing than I was on that score.
I doubt you will need Three-Pointer, and you'll have realized this by the time you reach that point in your feat plan, but you do have room for it. I had Quick Pockets and Run and Gun on my guy. Quick Pockets adds a spot for caltrops, and Run and Gun is good since SMGs benefit from being fairly close to enemies initially and in various situations.
I recommend speccing Blindsiding +5, Commando +3, Spec Ops +2, Opportunist: Slowed +3, and the other 2 wherever you prefer. Expertise SMG does benefit greatly from the boosts that Blindsiding provides, but you don't want to go whole hog on it since it is only useful on the first turn; but the first turn is often very important for Spec Ops SMG. Opportunist: Slowed is just a straight-up damage increase when you have Suppressive Fire, and is a no-brainer.
That build looks a lot like the sort of build I'd create, and is quite close to the build I actually ran, which means it's obviously a fantastic build that will obliterate everything that moves.