Wanting to try a red dot, I got a slide for my gen3 g19 from Aim surplus. It's been failing to go into battery like crazy. Some ammo is worse than others, shooting fiocci 124 grain it was shoot, hit the back of the slide, shoot, hit the back of the slide, for every shot.
Anyone had this happen? Is there a diagnosis or should I ditch the slide and go back to factory slide?

If it's one of the cerekoted ones, they tend to need a break in period from my experience from the added build-up of the coating.

It is cerakoted.  That was my thought, Maybe it needs a break in period. I went down today with 3 boxes of ammo,But it got so frustrating having to hit the slide every 2 or 3 shots I gave up before shooting it all.

That's one reason I went with the DLC instead, cerakote can reduce critical dimensions like slide rails.  Mine's run 100%, passed 1,400 rounds yesterday.

I'd say lap them with very fine grit sandpaper and a needle file to speed up the break in process.

Mine has exhibited some peening in the middle, where it contacts the locking block, so that may be a source of friction too.

Are you saying that the barrel locking lugs are peening?

If so, the barrel lugs should not peen if the barrel is properly heat treated.

Sand paper and needs files are really not the correct way to lap a slide to a frame. Use lapping compound instead.

For the OP,  It could be excess Cerakoting or it could be tolerance stacking causing issues. Also double check that the barrel is properly fitted to the slide. You want the top of the barrel hood to be flush or just a few thousands of an inch below the top of the slide.  

PS. you didn't say what frame you are using this slide on.

I have not had good luck with Aim Surplus parts or barrels. I bought a G23 9mm conversion barrel and I had to hand fit it because it sat way too low in the slide and would bind. Plus I never could get the barrel to function reliably. It caused all kinds of failure to feed and failure to extract issues. I tried the barrel is two OEM G23 slides along with a couple of aftermarket slides and it made no difference.

I have it on a gen 3 g19 frame.

No we're talking about the slide, the slide itself has slight peening, where it contacts the top ledges of the locking block at initial rearward travel.

1500 grit sandpaper wrapped around a needle file is just fine for that lapping/fitting process.  Not talking 60 grit.

A properly heat treated slide should not be peening either. And if a file by itself will remove material then the slide is not heat treated properly.