To Rick and HWL:
I have been shooting and reloading the .405 for my 1895 Winchester for a few years and have searched the internet. several loading manuals, and other sources for useable data(more on that later). The Hornady factory ammo worked fine on hogs and deer sized game, but not so well on larger game with big bones such as a Nilgai shoulder shot that did not shoot through. I immediately went to VV N133 powder and North Fork bullets for tough game and DG. When testing the first batch of hand loads, I saw that at the same Hornady published velocity (2225 on the ammo box) my loads were accurate and hit the same point of aim as the Hornady, allowing the same sight setting, so I stopped there - 2225 -2250 fps. Some 1895 hand loaders have taken the 300 grain bullets up to 2350-2400 fps and in the Ruger number 1, they go somewhat higher.
I acquired my .405 double December 2015 and have found that the 1895 loads work well in it too. After some careful shooting, I learned that each barrel shot minute of angle, but NOT into the same group.
Therefore I had gun maker Aaron Little regulate the double for me using factory Hornady 300 grain ammo(same base as my 1895 .405 for which I have a lot of data and ammo).
WRT load data in addition
WRT your Belgian doubles; since they were made last century, the ammo velocity may have been at or below the 2200 fps of the original Winchester 1895. Or your guns may never been regulated to shoot tight groups. Whatever the case, I can see two options:
1. Play with 300 grain loads above and below 2200 fps to try to find a velocity that does regulate to your satisfaction or
2. Have Aaron regulate the doubles to your specs ( I recommend sticking with the Hornady velocity) . Remember the old saying that "only accurate rifles are interesting" ? Regulation may cost several hundred dollars, but it made all the difference in my case.
BTW, the same 400 grain loads that shoot well in my 1895 also shoot minute of angle from each barrel of my double at 2029 fps (entry level of accuracy on 1895 too) and I plan to test more loads up to 2070 fps in my double. Now that the barrels are regulated to the 300 grain ammo, I am optimistic that I can find a 400 grain velocity that will regulate.
WRT load data in addition to those listed above by HWL, there are manuals by Hogdon, Ken Waters, Sharpe as well as lots of Load data on the Internet. I began using N133 for most of my loading after I learned that it typically produces 10K-12K psi less peak pressure than many other options; it also meters and compresses almost as well as ball powder. I like the Hornady manual because they have .405 loads using N133 as well as other powders.
Note- when I switched to Federal Large Rifle Match Primers, it added 20 fps to my 400 grain loads. This put my top 400 grain velocity well above the Hornady 2050 fps for their 450/400 ammo.
Rick, I second the request for pix and more info on your Belgium double.
