An interesting aside! I tried loading down the IMR 4198 and went down to 38gr and 14gr dacron. Group lowered-regulation acceptable-BUT PRIMERS POPPED SLIGHTLY AND SHOWED EXCESSIVE PRESSURE.
I'm not surprised. The 48 grain load sounds about right. The 38 grain is way too low. Primers backing out due to substantially lower than standard pressure is common in double rifles.
One of the most common complaints I hear about vintage double rifles is that they shoot way low or high with the original sights. It's usually load or sight picture. Keep in mind that the sights for your rifle were probably filed in for 270 grain, not 300. You might want to try 270 grain. That will make some difference in elevation, but not as much as you're seeing. Many of these older British rifles were set up for a six o'clock hold on the bottom of a 6" bull, with the bead buried in the bottom of the V.
If you're not able to solve the problem, have J. J. Perodeau make a new front sight, and keep the old one. I never file the rear express V on a prewar British rifle. Change the cheap part.
Can't imagine wanting to shoot lighter than standard loads in a .450 BPE as opposed to a .450 Nitro Express. But maybe I'm just used to Nitro Expresses.
