Ok, we've all read it, seen it and some have proved it. I talking about the

4% more powder = 1% velocity gain

My question is: can someone explain why. I wouldn't expect a 1:1 ratio but I WOULD expect it to vary based on case sizes, and bullet weights, and other thing.

caloric content of the slower powders used per grain is part of it. Usually you go to the next slower powder on the chart as a start. Even if you stay with the same powder, the fps/pg Feet per second/per grain ratio is a diminishing one. Otherwise we would see going from a 30gr case to a 60gr case would double velocity at the same pressure. Basically what I have been told is that there is a certain point at which the amount of powder being burned stops being a progressive even burn rate and some of the extra powder granules burning "bump" into each other and the energy is wasted. It is like they are burning and almost "clogging" in the neck and throat. That is why extending the throat is such a useful tool. The answer I got from a gentleman in the R&D at Picatinny Arsenal.

Rich
Rich

it's generally 25% of the increase in capacity. You can look at the various 300 magnums, at the same pressure.

UNLESS you change the case shape.. for example, the 22hornet AI is FAR more efficent than the case change, at the same pressure.

Or the 458 AR, with a shoulder and neck, is more than 1% better than the lott, as it has a shoulder to "jet" the gasses

jeffe

quote:

4% more powder = 1% velocity gain

It will vary some but for the 06 based cases it will be about 4 to 1.

When I built my 280 wildcat I plotted velocity vs capacity. starting at a 7-08 working up to the then very wildcat STW. With a 7mm bullet there are two lines one from the 7-08 up to the 7X61 S&H that line increase at about 1 to 4. Above 7X61 the velocity starts to flatten until you almost have a flat line. At the STW and Rum level there is almost no gain for more capacity. I designed my 280PDK to give me the capacity at the intersection of the two lines. IE the maximum capacity before the line starts to flatten. In an off the shelf the 7X61 S&H is about as close as you can get to that point.

dfaugh - it does vary by case capacity. Not so much by bullet size, until you get into very light or heavy bullets for the caliber (look at the Ft/lbs. of energy for the different bullet weights, they all should be about the same. Very light/heavy bullets will have the energy levels drop compared to a more appropriate bullet weight for the caliber. The light bullets drop because they move so easily the total volume (chamber + barrel behind the bullet) increases so fast the total area under the pressure curve drops. You need faster burning powders, but then you can't use as much powder before hitting peak pressure so total energy goes down. Too heavy bullets just use up too much chamber space, so you don't have room for the powder; again total energy goes down.

The 4:1 ratio is general because it roughly works for 308, 30-06 and the 2.5" case magnums like the 7mm Rem magnum and 300/338 Win magnums. These are the cartridge cases most used and the ratio works well for them. Go up to the 505 Gibbs case, and it's more like 6:1, you need 6% more powder to get 1% more velocity. Go down to 221 Fireball and 22 Hornet cases and it's below 3:1; a 3% powder increase will give more than a 1% velocity increase. Look at tiny cases like the 30 luger and 30 mauser, very little powder can give you a significant velocity increase.