So many Enfields have enormous groove diameters. I owned a couple of them years ago with this issue and leading and tumbling bullets were a problem. At the time I was using (now discontinued) SR4759 and the solution for me was a filler between the powder and the bullet, I used PSB or Precision Shot Buffer. As per instructions I found here I added the weight of the buffer to the weight of the bullet and reduced the powder charge accordingly.
I cannot explain how or why the buffer solved the problems, but it did. You might want to do some research here regarding the .303, Enfields and fillers.
35W