I'm new to this forum and double rifles, but recently acquired a 450/400 3.25" NE (a Rhodda full sidelock, vintage 1908)and want to assemble some loads for it. I've read all of the threads I could find in this forum, plus the Seyfried articles in DGJ, and have slugged the bores, which are clearly .408". I plan to start with 400 gr. Woodleighs and about 66 gr. of Rel 15 and work up to the 1.19 formula or until I get left/right regulation and velocities in the 2100 fps range, or pressure signs. I also want to test the "75% rule" that NE 450 No 2 has written about using the same powder charge, 300 gr. Hawks, and dacron filler, the intent being to create a milder pig load. Am I going about this correctly? Should I use filler in the 400 gr. load (it looks like there will be about a 1/2" gap between powder and base of bullet)? What is the recommended OAL of the cartridge? Thanks.

Let us see a picture of your rifle!

I have a .411 bore .450/.400 3 1/4" Westley Richards. My first load was 81 grains of IMR 4831 and Federal 215's (which is published in the A-Square manual). No filler is needed with it and my first four shots were dead on, so I just stopped right there.

The A-Square loads have pressures listed and that could be invaluable in your work ups...

Judge's load is a good one, and is one of the loads I shoot in my 450/400 3 1/4".
My rifle also shoots good with 71.5 gr of RL-15.
I do not use filler with IMR 4831, but you MUST use it with RL-15.
Since you are already aware of my 75% theory, I will give you another good light load I have found. I use the same 300 gr Hawk m.025jkt bullets, with 46 grains of IMR 4198 [with filler]. This is a Nitro for Black load that should have a velocity of 1700 to 1800 fps. Very mild recoil. I am using foam as a filler. I cut it out with a 50 BMG case.
I seat the Woodleigh's to the cannelure.

I settled on 71.5 grs. of RL-15 and about 5 grs. of dacron in my 450-400-3.25 and it worked like a charm..Leave no space in a double rifle or you could ring your chamber or worse has happened, thus a good tight filler.

There are pros and cons on the use of IMR or H 4831 in double rifles, by some very experienced double rifle experts, some of them claim it is a dangerous practice as the the pressure stays high too far down the barrel and it has apparantly caused some split barrels and hadn injurys...I have used it a lot without any problems what so ever, but I saftied up and went the RL-15 route just in case the knew from whence they came...

Thanks for all of the help. I'm assembling some loads using Rel 15, 400 gr. Woodleighs, and filler. Will report back after a session at the range. Will also post a few pictures of the gun if I can ever figure out how to get "permission" to do so on this forum.