The factory 405's tend to open up well but may not hold together at upper velocities {over 1600 fps} depending on what you hit; shoulder, etc.

I am not promoting the following load for your double at all, since I do not know what pressures it generates, but Daryl is right as far as my Marlin is concerned; 45 grains of 3031 + grease wad column making for a compressed load, shooting the Lee "400" grain Hollow Point {mine weigh 402 grains cast of wheelweights} go 1640 fps, spot on to what Daryl says here.

It is a killer of a bullet at that speed, too. I have killed bear, deer and 2 range cattle with it. I do not know if they still make the mold but it in my opinion makes for one of the best .45-70 bullets ever made for big critters or small as the bullet acts just about like a Nosler Partition, expanding fast and then blowing off the HP portion with the base shaft of 300+ grains continuing on. One of those range cattle was shot in the head as it came for me and the bullet penetrated a later-measured 24 inches of bone, grislte and muscle, shattering neck vertebrae the whole way and coming to rest in the mass between the shoulders. In my test media it has performed similarly at close range to Swift A-Frames in the .375 H&H Magnum.