Herbert,
Go to Accurate Molds online. Go to "Products and Prices" then to "configure a mold" in the left bottom of the "molds" block and from there you custom order the mold you want , you will select mold body metal, for a machine or hand casting, the # of cavities, sprue cutter type you want and mold number (write down the mold # you are wanting before you go there so you don't have to go back and find it.).
There you can also make any mold changes you want like : diameter, bullet length, grove depth, ect... that you think would be better for your gun barrel.
The mold you are basing your mold on is: 47-265S.
You will need to add about 25% to the length of the heel of the bullet to get to the 300-320grs you are wanting or trim you brass shorter
in order to add length to the bearing surface on the bullet. Make sure that you leave enough throat on the brass to hold the bullet heel straight and firm even after crimping or you can cause it to become misaligned during the stress of feeding that will make it prone to jamming.