Mike:
I owned a Belgian .405 DR for a while and I never did get it regulated. The rifle was of a quality level similar to yours. I believe the name on it was G. Schraff, but I'd have to check my records to be sure.
This was before the recent availability of new factory .405 Winchester ammo. I bought a box and a half of original .405 ammo at some ridiculous price and ended up with two beautiful 2 inch groups on each target at 100 yards. Unfortunately they were spaced 18 inches apart.
I spent more than two years working up handloads of almost every stripe. Nothing ever got the two barrels closer together than 8 inches and I shudder to think how hot those loads must have been.
To my knowledge, there was never a factory .405 load of any bullet weight but the 300 grain. Hawk Precision (or whatever they are called today) was offering lighter bullets (250 or 270 grain - can't remember which) at the time, but I didn't get any better results with the Hawks than I did with the other stuff I was handloading. I finally gave up on the rifle, consigned it to Pete Harvey, who told me it went to Alaska as a carry rifle for some guy who liked to salmon fish in places where a lot of his fellow fishermen wear heavy brown fur coats even in the summer heat.
Although it's counterintuitive, I've had some success with a light .250-3000 double rifle moving the barrels together using 120 grain bullets at about 2400 fps in place of the factory 87 grain loads at 3000 fps for which the rifle is proofed. I have no idea why this worked, as it's contrary to everything I've been told about regulating a DR. Even so, you might try slowing the 300 grain bullets down to 2000 fps or so and see if it changes anything.
Let us know how you make out.
Rem