I had the same issue when I was shooting retumbo. I found my 300 rum was fouling out after about 12 round and accuracy went out the window. I then switched to Hodgons H1000 and havent looked back. still slightly black around case mouth but I can shoot 25+ rounds with excellent accuracy. Retumbo IMO is just a littte too slow for the 300 rum. gook luck hope this helps.
ps. my current load is 91.5 grns h1000 w/ 200 grn accubond = 3100fps from 27'' lilja barrel
" People sleep peacably in their bed at night only because rough men stand ready to do violence on their behalf " - George Orwell
I am the way I am because I watched the movies "Red Dawn" and "Tremors" at a impressionable age.