.45-70 pressure question

[VVelox]

So the question of power for of .45-70 Gov. v. .458 SOCOM comes up fairly often. This got me curious as the what is rated for a higher pressure as they both take the same bullet.

For .458 SOCOM, it is 35k PSI, but unfortunately the .45-70 Gov. is measured in CUP, meaning there is no direct conversion for all I've found. It is listed at 28k CUP. Any one know of any official PSI value for modern .45-70?

Although the Denton Bramwell equation does suggest ~42k PSI is close.

 

[Reloadron]

If we use this chart the 28K CUP for the 45-70 Government works out to be 28K PSI for the 45-70 Government.

However, there are several caveats to consider. For example looking at the 45-70 loadings in the Lyman 49th Edition Loading Manual the loadings for a 1873 Springfield Trapdoor remain well under the 28K CUP for a 300 grain PP jacketed bullet. Now if we get into loadings for the 1886 Winchester and 1895 Marlin new production lever guns using a 300 grain Jacketed PP bullet we have loadings in the 26K and 27K CUP ranges. Moving along to the strong action Ruger No 1 and No 3 rifles we see loadings in the 37K to 38K CUP ranges, these well exceed the 28K CUP specification for the same 300 grain jacketed PP bullet.

The 45-70 when it was born was made as a black powder loading somewhere around 1873. It is a very large case. Many of today's firearms can handle the higher pressures when the case is stuffed with the new smokeless propellants.

Keep in mind that with the 45-70 the 28K CUP converts to 28K PSI but only at that point. So if I load a 45-70 Government to a pressure of 37,300 CUP that does not work out to be 37,300 PSI.

Also, as to "power" loading a 45-70 Government with a maximum suggested load of 55 grains of IMR 4198 below a 300 grain jacketed PP bullet I can pretty much expect a muzzle velocity well in excess of 2,000 FPS for that big heavy bullet. To my way of thinking a comparison between a 458 SOCOM and 45-70 Government is sort of an apple and orange thing. The 458 SOCOM was designed to meet certain criteria including the ability to fit a specific magazine design.

That is my explanation anyway and I sure as heck might be wrong. Hopefully another member will have more to add.

Ron

 

[REB]

Hodgdon list modern loads in 45-70 up to 44.6K PSI and lever action loads up to 38.7K PSI.

The problem is I don't think there is really a standard for these modern 45-70 loads

 

[Don McDowell]

Pressure really doesn't mean diddlesplat when it comes to matching velocity sometimes.
For instance the 454 casull throws a 300 gr bullet at around 1600 fps at something over 50K psi. The 45-70 throws the same 300 gr bullet at around 1600 fps at somewhere around 14K psi when loaded with a case full of blackpowder.
Another prime example for pressure and velocity comparison is between the 308 and 30-06.
Velocity is what counts when trying to figure out power equations,

 

[Gtscotty]

Pressure really doesn't mean diddlesplat when it comes to matching velocity sometimes.
For instance the 454 casull throws a 300 gr bullet at around 1600 fps at something over 50K psi. The 45-70 throws the same 300 gr bullet at around 1600 fps at somewhere around 14K psi when loaded with a case full of blackpowder.

Velocity is always directly related to the area under the pressure curve, which is, in turn always related to pressure levels in any given part of the firing process and how long those pressures are maintained. Although you can sometimes extend the pressure curve with large amounts of show burning powder to achieve more velocity, you cannot separate pressure from the velocity equation.

 

[Don McDowell]

Well I'm going to say it might be best to look at data contained in places such as Lyman's books. If you look say at the 30-30 data, and take close notice of the velocity and pressure data for the same weight bullet in jacketed or cast, you'll see that altho many/most of the cast bullet pressures using "pistol" powders have the same pressure as the jacketed loads using "rifle" powders, the velocity of the cast is approximately 3/4 that of the jacketed.
Trailboss powder is another prime example, specifically if you look at the 45-70 data. That stuff reaches firewall pressures (saamispec/trapdoor) yet at best can barely make 3/4 velocity using black and sometimes just a little over half of the velocity using a host of smokeless powders.
30-06 vs 300 magnum..
Just because there's high pressure doesn't necessarily mean there's the velocity to go with it.

 

[Gtscotty]

Exactly, it's about the area under the pressure curve. Even though the pistol powder loads have the same peak pressure, the duration is much shorter. Significant force is applied to the base of the bullet for less time, resulting in less velocity. None the less, If two loads have similar expansion ratios, powders and bullets, as a general rule, the round with the higher average pressure is going to have more velocity.

 

[Gtscotty]

It looks like the .458 socom has around 61 gr case capacity whereas the 45-70 has around 79 gr case capacity. The socom is going to have to be run at a higher average pressure to rival the performance of the 45-70, all other things being equal.