What Buffer Should I use in my KP-15?  
We do not have data for every gas system length, gas port size, and ammunition type to definitively tell you what you should use in your complete system and in your environmental conditions.  This is ultimately going to be up to you to determine by observing the ejection pattern and testing for last shot lock back.  Generally run the heaviest buffer you can that still cycles reliably.  

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-15 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.

Reference the graphic below for tuning your system.

The JP Silent Capture Spring System and Unrivaled buffers are approved for use. Use whatever combination of spring and weight needed for your rifle to run reliably.

Pistol Caliber Carbines
9mm Blowback systems require a minimum buffer weight of 5.4oz to not damage the lower.
.45 ACP Blowback systems require a minimum buffer weight of 8oz to not damage the lower.

Rifle Length Buffers and Springs will NOT fit.  You do not need a rifle buffer to use a rifle gas system: