in nitro-doubles I would rather go with another formula I once read in an old double-rifle catalogue-reprint that puts the weight-question on the point:

bullet weight (grains) multiplied by velocity (fts) divided through factor 250 = minimum rifle weight in gramms

for example: .500 NE: 570 x 2150 : 250 = 4900 gramms = 4,90 kg
.577 NE: 750 x 2050 : 250 = 6150 gramms = 6,15 kg
.450 NE: 500 x 2150 : 250 = 4300 gramms = 4,30 kg
.600 NE: 900 x 1900 : 250 = 6840 gramms = 6,84 kg