What Buffer Should I use in my KE-9/KP-9?

The minimum buffer weight to not damage an aluminum or polymer lower with 9mm blowback system is 5.4oz.  This is what all of our complete lowers ship with.

In the KP-9 Most importantly DO NOT USE

Flat wire springs
Braided wire springs
Captured spring systems with sharp edges.
All of these can damage your lower.

The KP-9 receiver uses a Carbine buffer and spring.

We are only referring to length with the word “carbine” here, not weight. Use whatever weight of carbine buffer you need for reliability and smooth cycling. Use whatever weight of carbine spring you need, so long as it is a standard round spring.

Are extended heavier buffers ok in the KE-9/KP-9?

Yes and we use them ourselves. The bolt of a 9mm AR does not need to travel as far to successfully chamber, extract, and eject a casing. You can accordingly use longer heavier buffers in 9mm blowback systems than would fit in a carbine length buffer 5.56mm 

We personally use 8oz-10oz buffers with .308 or Sprinco orange springs in our own KE-9s and KP-9s, but this combo will not work for every shooter or for lower powered ammunition.  it is possible to "limp shoulder" with this buffer system if you are not holding the gun solidly and roll with the recoil too much, though it does feel more pleasant to shoot.

Rifle Length Buffers will NOT fit in the KP-9, and using one in the KE-9 would require a rifle length stock/buffer tube to be installed.