The fastest I have hit is just over 5,000 fps. Three shots over 5,000 averaged 5,006. Ruined the brass in the process. However I'm in the rare 5,000 fps club. one that PO. Ackley tried and tried to get to, but he didn't have Hammer Bullets. Would I chase 5,000 fps again? No it is too hard on the barrel. I've been hanging around Joe too much.
Jesse to be realistic, I think 4,200 range for the 124HH is a reasonable goal. I also think anything between 3,800 and 3,900 for the 137 or 138 is excellent. I have an accurate load at 3,849 out of my rifle. However, I have a 26 inch barrel and those extra 4 inches with all that powder makes a difference.
3899 fps is smoking! It tells me that you have a real winner. I'd back that load off by 2 grains and shoot for accuracy.
This week I'm planning (I'm off work!) to get a nice accurate load with the 154HHT. The nice thing about having a RUM and reloading is you don't always have to run it at full speed. When I was into hot rods, the old saying, "There is no replacement for displacement." is also true for powder capacity. If you feel like you need more, you can always get a 30-378 Weatherby Mag. It holds 17.7 more grains of water than the RUM.