The Hornady Manual states that 45-70 loads for the Marlin are held to 40,000 lbs which is the same max pressure as the 444 Marlin. The Sierra Manual lists the Marlin with the Ruger No. l and 3 and the converted mausers. Hornady has always held that the Marlin action is safe to 40000.

The Guide gun would loose some velocity but its really not an issue at the ranges that it is intended to be used. Max penetration of the big cast bullets is attained at from 1600-1800 fps. Bullets beyond those speeds began to loose penetration. This was found in Corbons study with cast bullets. I should say either corbon or Garrets study I don't recall which. I have used the 45-70 a lot over the last 30 years.