I am using RL15 for those 310's as well. I was only pushing them 2250-2300fps. The slug was found in the 8th milk jug, having cracked the back - I measured it at 4' to the back of that jug.

I also shoot the NOE Thumper in my Whelens. I use WC852 or WC 852f surplus and worked my way up to 55 grains of WC852f. I water dropped them and used 2500+ lube.