Advice needed on project and bolt thrust.....
My mind is always on the go so I noticed a gun and saw a conversion and now have alot of questions.
The gun : Navy Arms 45-70 using a Enfield #4 action.
Conversion: 45-70 to 45-90
The idea: The action is good for 49000-52000PSI. The reason there is a range rather then a single number is the difference between the 303 and 308 they are chambered for. The gun is rated to take some of the hotest loads made for the 45-70. If I ream it out to except 45-90 I should be able to get 2-300 fps more out of her, and keep the pressures about the same. Now the 45-90 and 458 Win Mag is almost the same, as far as what it can hold powder wise.
The question: If I keep the pressures around 50000psi, I should be able to get 400 grains to 2352 fps or 450 grains to 2200 fps. The question is with bolt thrust. The rear if the cartridges are about the same and so would be the pressures. Would I have a problem with the conversion and numbers?
Now I know most folks would say, "why not just buy a 458 and not worry about it" but I like to do projects every once in a while. Your thoughts......
I wish Murphy still hung around for more definitive answers. All I can offer is guesswork and comparisons.
The guesswork is that with the extra case length, you'll be getting more "grab" from the case on the cylinder walls which will offset some portion of any extra thrust. On the comparison side, I've got Sharpes in both 50-70 and 50-140-- same action and all. The 50-140 just about knocks you senseless compared to the mild recoil of the 50-70. I push 650 grain bullets in the 50-140 to roughly the same velocities as 450 grain bullets in the 50-70. I've never tried to "magnumize" the 50-140 by pushing the 450's to higher velocities, but I have read that it be done. Just not when pressed against my shoulder.
I want to have fun and just not push a bad idea.
From what I can find, the only two variables needed to calculate bolt thrust are internal case head dimension and chamber pressure. That being said, bolt thrust should be the same with either cartridge loaded to the same pressure.
Here's a link to Lilja's page that talks about bolt thrust, as well as lug shear and bolt flex:
Max chamber pressure times the internal area of the case at the head will give max bolt thrust (minus the trust taken as tension in the case). So basically bolt trust is a function of max pressure.
What the larger case will give you is more powder room so you can use more of a slower powder, which will allow the pressure to stay higher longer even if it has the same peak value.
The issue is how much powder room you have, and how it is effected by the max overall case length that will feed. I have not looked at the Navy Arms version but did look at them back when these Enfield conversion were offered by Summit. What I could find then was that seating to max overall length, there was no improvement in powder room with heavy bullets. The advantage in converting came when using light 300 and 350 grain bullets which could not be seated out far enough with the 45-70 case to take advantage of the overall length the action could feed.