So i have a can with 9 Stainless Steel K-Cups like these; I am planning to use if for a 9mm AR9, trying to see how i could make it perform better.

Any ideas?

Putting a hole in it will help.

Yeah well other then that. They are getting drilled to .422"

Don't drill them that big.

Search for K-cup clipping.  You'll find lots of info.

K-baffles (integral orifice plate and conical spacer) are usually finished with a ball end mill to put a scoop into the baffle face at the edge of the bore. Then cutting from inside the skirt an opposing back-scoop and port.  This promotes a cross jet into the annular space formed by the skirt exterior and rear face of the plate.  Hand filing or power tool techniques have a difficult time forming these two cuts.

A “dater hole” in the face of the blast baffle can help a little with first round pop and lowering blast chamber pressure a little quicker.

The most quieting features create cross jets or turbulence to move gas off the bore line.

Here's a typical cross-section. This one includes the Dater hole, KP referenced.



This is somewhat how the face should look. A concentric hole, bored thru, with an offset "scoop" or clip.



When setting up to cut the scoop, you need to factor where to index the cutter's center.
Too deep or close to centerline will allow gas to slip-by.



You're trying to align it so the bottom of the cut's edge, closest to bore centerline, creates a sort of lip, which redirects the gas across the boreline and through the vent cut.



I prefer to cut the vent from the front, and angled from the central axis. I've always had good luck with it and it just makes more sense as that's the path you want the gas to flow.



Cut the vent shallow, you can always make it larger. The edge will be thin and a diamond needle file will be able to help enlarge it w/o needing to replace in mill.
This is what you're going for, from the outside, looking at the coaxial volumes and vents.

Or, and just a thought here, maybe the best way to make it better is to find a SOT to recore it for you with internals better suited for your use

Clip them all

Where are those available?

In 2021

STC K-Cup