So after 2 years of building a rifle and developing a load I have finally achieved success. Since 300 smks have been next to impossible to find in the past 6 months I had to develope a load using Hornady 285 gr BTHP. After much research I had turned up next to nothing on loads for the 388 RUM using this particular projectile. I did what most reloaders would do and found a smiliar weighted bullets load data and started with the absolute minimum powder charge and worked up. I used H1000 because it was readily available at my local cabelas. I started somewhere around 87 grains and ended up going up to 97 grains in half grain increments. 97 was way too hot and 96.5 was having some extraction issues. Here is the final load I have settled on:
Remington 700 with a 26" GAP barrel 1/10 twist 5R rifling.
96 grains H1000
CCI magnum primers
285 grain Hornady BTHP
Keep in mind that I was able to load this round much longer than all the books tell you. Therefore increasing powder capacity and decreasing pressure. DO NOT attempt this load at the saami COAL spec of 3.6 for obvious reasons.
I'd like to thank my buddy Mike for helping me with this very long process.
Now I just need a muzzle brake