Re: 300 RUM or 300 Jarrett
I had Kenny build a .300 Jarrett for me a number of years ago (it is one of the "Accuracy Legend" series). The rifle has a 24" 1:12" #4 contour Jarrett barrel (one of his first) with a brake, and it will shoot a 165 Ballistic Tip at 3450 fps and a 200 gr Partition at 3125 fps with 82.5 and 78 grs of RL22 respectively. Accuracy when new with the Ballistic Tip was sub 1/2" and the Partition was sub 3/4" for 3 shots. I haven't shot a group with it in some time so I don't know what it will do now.
There was no .300 RUM when I had this rifle built and I hear what everyone is saying. They are probably right and I may have reached the same conclusion had it been available. However, the .300 Jarrett is a pretty good choice and although it is a wildcat, you are not as out of luck as you think should your ammo not arrive. It will fire both .300 Weatherby and .300 H&H with no problem. The Weatherby ammo is at least as common as the .300 RUM ammo so there is no difference from that standpoint.