JERRYO:
"Perhaps I used to many words.

You asked what is different between a .300 mag and a 45-70. I answered that the 45-70 bullet has more momentum. This momentum is equaled by the gun.

I was trying to point out that frame stretch in a 45-70 would be affected by the momemtum and the distribution between the barrel and the frame.

So a 45-70 rifle would stress the frame more than a pistol as it has a heavier barrel that must be pulled by the frame.

The 300 win mag will have considerable forward push on the barrel from the pressure on the shoulder, but less momentum."

Thank you for not using too many words this time. You made a good point just like there are many points that could be made. I am sure yours carry as much weight as mine and the rest.
Unless I am inaccurate in my subject matter the original question was concerning PSI not recoil. The Encore is chambered for many cartridges including the 458 Win Mag, 458 Lott, 470 Nitro Express, and 50 Alaskan to name just a few.
Now if we were to talk about recoil and its effect on the frame then we would have good subjects to pontificate on.
That is what I meant by my post saying there were so many varibles that we would never end this thread if we discussed them all.
With some their effect would be hard to even discuss in a knowlegable way just as your momemtun idea verses the push on the shoulder of the 300 Mag. Would the momentum of the 45/70 be no worse than the shoulder area of the 300 Mag pushing against the front of the chamber? In other words would the effect of one cancel out the effects of the other?
Then we could ask if some one of great physical strength was shooting the 45/70 and some thin weak person was shooting the 300 what kind of difference would that make in how the momentum affects the frame.
This is one of those threads that really answers very little. It only serves to bring up many, many, many, many, many, many, many, many more varibles that is difficult to prove or disprove.
You know why? Because there are a lots of varibles. Did I mention that?

Have a good day.

PS: I asked why if the Encore can take the 300 win Mag at 64000 psi why couldn't it take the 45/70 at 50000 psi. I don't beleve the original poster mentioned the recoil thing.

Glad I could help.