I don't have a load with heavy bullet in 303. but here is one in 7.62x54R Mosin-Nagant. Lyman 313299 weighs 200grs and the Mosin twist is 1 in 9.5 against 1 in 10 in any Enfield. Too close to make a difference.
My only experience with fillers in 303 was with 16grs of Vihtavuori N140 (40% of normal load with 174gr jacketed bullet), topped with 22grs of Semolina, so that there was no air space below the bullet. I fired that the following day, so there was no chance of semolina picking up moisture and clumping. Accuracy was superb. All in in about 1MOA and so quiet that the boys thought I was firing primed cases! But the rifle got very hot after only 10 rounds and the cases were difficult to extract. The problem is that semolina compacts into a solid plug under pressure and firing it in bottle neck cases is not good for your health!!
Regards,
Alex