I agree with MkVII on this one; rain will up the pressure and it's only sensible house keeping to keep ammunition as dry as possible, I have shot at Bisley in pouring rain next to another club who were using converted No4's and not taking the slighest interest in keeping ammunition dry, and that is not a comfortable feeling!
LERA's take on this is that club 7.62mm converted No 4's will only be used with 144 Grn ammunition, and as TR Captain I believe that this is a sensible course to take. I have reloaded 150 Grn Sierra Match Kings for No 4 actions but would not (my choice) use 155 Grn bullets.
We have to be accountable for our actions when sharing range space with other people, however it does not mean that I agree entirely with the NRA statement which I think was badly researched and worded.