Staying within reloading manual guidelines.

Looks like the velocities I Like get out of my 416 Taylor.

My first reaction is that you should try to find data for slower powders with lower bulk density, like Ramshot Hunter, Norma 205, H450, Hodgdon Superformance, Accurate MagPro, Ramshot Magnum (from faster to slower). The Rigby is also very close to the 416 Weatherby in volume, so data for the Weatherby could be of some help as well.

My Ruger M77 loves 400 gr. bullets and H4831. The old Swift A-frames will group around 1" at 100y. Velocity is right at 2500 fps. Interestingly, I had some surplus IMR5010 so tried a case full and got 2300 fps and again groups of about 1".
C.G.B.

Ive never had any issues with a 400 grain TSX/TBBC/A frame at 2400 for everything out of a .416 Rigby.

No issues getting it to 1 MOA for a 5 shot group- when I had the old barrel on mine, I chased fine accuracy and came up with a .5 MOA load for it- then shot the barrel out (it lost accuracy in the middle of a hunt...) and now I just use that same load in the new barrel which gives the 1 MOA.

I've shot stuff out to 300 yards with it. Minute of Eland out to 300 from field positions, and it killed a grysbok at 25 yards.

My issue with it is it is more gun and more weight than I really like to lug around in North America.

That and I enjoy playing with rifles. Don't want any of them in the safe getting neglected, after all.

I have been using the CEB 340gr single load bullet in my 416 Rigby. Can't find any reference to V15 but I think it's the same bullet you are using. My gun has a 1-14 twist so maybe it works a little better.

Forget about the 47K pressure limit unless you are using some rifle made around World War I. As you indicated it is hard to get a case full of the proper powder without exceeding that pressure. I have been using 101 grs of 4831SC with that bullet and I am still way below max. Whether I will get there I don't know, my shoulder will probably give way long before my rifle.

BTW Saeed has posted he is using 105grs of H4350 in his 416 Rigby and 416 Weatherby with 400 gr bullets, with no problems in either.

I used H4831 and IR-4350 exclusively, with 400 gr. softs and solids but I only used big bores to shoot buffalo and such at 25 yards, so didn't get in too deep in what the gun liked or didn't like, and a 2" group was fine but most big bores shot and inch, Saved that work up for varmint rifles, and mid caliber to some extent in that a one inch group was my goal..

The 416 Rigby will get some awesome velocity and they hold up well to a 400 gr. bullet at 2700 fps, Ive seen that load used many time and even at higher velocity by our hunters over the years, but that said bullet failure sure was a problem..The best DG combo is a 400 gr. bullet at 2400 FPS on DG of any kind, both soft and solids..I still like a round nose soft 450 gr. woodleigh at at 2350 fps..and a flat nose solid if the gun will feed them. if not a RN suits me.

The 416 Rigby and the 404 J. are all but identical and have the same powder capacity I have read and holding a 416 Rigby and a wildcat 404 IMproved together it must be a fact..I knew some guys back in the day that shot their 404 with case fulls of 4831 and 4350 DATA with 400 gr. bullets at 2800 plus some? and the Rigby a 2900 plus some, and lived too talk about it no problems at all..Not recommending anything here, just what I observed, and as I recall mentioned water capacity was the same..They were the godfathers of wildcatting, and now gone to the big loading room in the sky..

Lyman, Hornady, and Nosler list loads for the 400 grain bullet.