I'd use a heavy Accubond and a slow powder.

ADI 2218 is Hodgdons 50 BMG.

It's where I would start:

Code
Cartridge          : .264 Win. Mag.
Bullet             : .264, 140, Nosler AccuBond 57873
Useable Case Capaci: 74.864 grain H2O = 4.861 cm³
Cartridge O.A.L. L6: 3.340 inch = 84.84 mm
Barrel Length      : 26.0 inch = 660.4 mm
Powder             : ADI AR 2218

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.649% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms

-06.5   98    72.00   3026    2847   48404  15353     96.4    1.396
-05.8   99    72.50   3053    2897   49612  15465     96.8    1.380
-05.2   99    73.00   3079    2948   50857  15573     97.2    1.363
-04.5  100    73.50   3106    2999   52139  15675     97.5    1.347
-03.9  101    74.00   3132    3050   53456  15772     97.8    1.331  ! Near Maximum !
-03.2  101    74.50   3159    3102   54814  15865     98.1    1.315  ! Near Maximum !
-02.6  102    75.00   3186    3155   56207  15951     98.4    1.300  ! Near Maximum !
-01.9  103    75.50   3212    3208   57648  16033     98.7    1.284  ! Near Maximum !
-01.3  103    76.00   3239    3261   59131  16109     98.9    1.269  ! Near Maximum !
-00.6  104    76.50   3265    3315   60659  16179     99.1    1.253  ! Near Maximum !
+00.0  105    77.00   3292    3369   62232  16243     99.3    1.239  !DANGEROUS LOAD-DO NOT USE!
+00.6  105    77.50   3319    3424   63854  16302     99.5    1.224  !DANGEROUS LOAD-DO NOT USE!
+01.3  106    78.00   3345    3479   65526  16355     99.6    1.209  !DANGEROUS LOAD-DO NOT USE!
+01.9  107    78.50   3372    3534   67251  16401     99.7    1.194  !DANGEROUS LOAD-DO NOT USE!
+02.6  107    79.00   3398    3590   69031  16441     99.8    1.180  !DANGEROUS LOAD-DO NOT USE!
+03.2  108    79.50   3425    3646   70867  16475     99.9    1.166  !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 3% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 3% relative to nominal value:
+Ba    105    77.00   3359    3508   66604  16102     99.9    1.202  !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 3% relative to nominal value:
-Ba    105    77.00   3217    3217   58105  16242     98.0    1.278  ! Near Maximum !

You didn't use logic or reason to get into this opinion, I cannot use logic or reason to get you out of it.

You cannot over estimate the unimportance of nearly everything. John Maxwell