I've had zero experience machining 420 stainless, but I have machined a bit of 316 and 304.
The heat builds up in the chip and the stuff has quite an affinity for welding itself to the tool, even a simple drill bit cannot be "rushed along" in stainless.
I think your feedrate is about 4 times too high. Another thing about a ball mill, is if you are using the bottom side of it, there is often not great rake available for the cutting face of the flute, and this makes it a b!tch to get the chip out of there, and another reason you cannot feed it hard.
I'd try a two flute, just for the fact that there is a little more room to work on grinding the flute and even grind a bit more rake face if necessary. I guess that destroys any coating the face had, but what else can you do?
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