284 Winchester brass, necked up to 30 caliber.
150 - 180 gr bullets. 174 gr work the best - these are FMJ pulls from military rounds, US, M2 ball.
Have yet to find a powder that does not work in of the three K31's that I own.
As you can see from this thread, I purchased Redding dies after trying a set of another brand. The Redding dies, so far, are the only dies on the market sized to the K31 chamber. 2 die set was almost $60.