in2anity wrote:SCJ429 wrote:When a 45/70 fires you increase the volume of the combustion area very quickly, the bullet travels the length of the case down the barrel to double the area to be filled by the expanding gasses. To keep the pressure up you need to burn the powder quickly.
you da man SCJ - thanks for that mate, makes good sense
I take it that's one of the benefits of a bottleneck? I.e. the combustion area is essentially "throttled" by the tapered-down neck? (thus better facilitating a slower burning powder)
Thats it exactly mate, as SCJ429 is saying, better expansion ratio in the straight walled cases. Bottlenecks are technically less efficient pistons, a lot of pressure behind a smaller area, meaning they need slower burners to move bullets. However the upside of the pressure on the smaller surface area is of course the higher velocities with nice slender aerodynamic projectiles.
The advantage of better expansion ratio or large calibre for the same powder charge is more power of course. You increase the calibre, you get a more efficient piston, more kinetic energy. There is a easy formula to work this out.
For example taking a 378wby case , and necking it up to 416, 460 and 500 cal, the maximum kinetic energy jumps. In this example it goes about 6000ftlbs, 6700ftlbs, 7500ftlbs, 8500ftlbs.
Basically work out the surface area of the base of a flat base bullet in both examples. If the bigger one is a certain % larger surface area, it will be half that amount more powerful than the smaller one.
Eg a 378 weathbery(375bullet) has a bullet base area of about.110 sq in, the 460( 458 bullet) has a bullet base area about .165 sq in. The 460 is 1.5x more area, so about 1.25x more kinetic energy.
You can also apply the same for powder capacity differences. If one case is 50% more powder than another, then it will produce half that amount increase in kinetic energy. So 1.25x. Try it comparing say 30-30 to 308 to 300 win mag.
You can also work out the difference now between two cases of both different size and calibre. Just do the multiplication for the powder, then for the calibre change, multiple the differences together. You can also work backwards to smaller cases.
Its rough but pretty close. Where its less accurate is smaller cases or extra long long bullets which might eat a little too much into powder capacities. its easier not to go off loaded powder data, just go off the cases total water capacity in grains which you can find off the net. Also where necking a case up and down reduces or increases the powder capacity, it may effect the result. For example you might get some more capacity necking cases up a great deal as it blows out the neck and shoulder angles too. You also need to assume same pressures used in each case, or at least a normal factory maximum.
Where is this random waffle useful? Probably nowhere
But it was to those of us into wildcatting new cartridges back in the day. If you wanted to know what the end result of saying doing something whacky like necking up a 378 wby to .550 cal for the first time you could project possible power levels. Or working out the max power of some older cartridge in a new action, like a 400 nitro in a ruger single shot back when people started doing it. You'd find a calibre roughly close in size, say the 458 lott. Measure the water capacity for both cases, calculate the base diametres, and then work out where max 400 nitro power will theoretically run based off the lotts maximum energy.
by » 03 Sep 2019, 2:31 pm
