Probably HS-5, HS-6, or HS-7. All older St. Marks spherical powders that are slower than the 231/HP-38 that is their powder that is popular for target loads. HS-6 is still available, and was once a popular full power factor powder, but Power Pistol and others have mostly taken its place. None of those HS powders is fast, and, in general, the slower the powder, the higher the pressure you have to run it at to get reasonably clean burning. Slow powder and low pressure don't go together for that and several other reasons.
Stubbicat,
Expansion and sectional density are the key things to understand. When the cartridge is assembled, there is a certain amount of volume under the bullet for the powder to start burning in. This is usually called the powder space. But once the bullet starts to move forward, the volume the powder is burning in grows behind the bullet as it moves. The shorter the powder space and wider the bullet, the less distance the bullet has to move to double the powder space, and if the pressure is to be maintained, the powder has to make twice as much gas by the time it gets there. If the pressure is to increase, it has to make more than twice as much gas by the time the bullet gets there. The faster the bullet moves, the faster the powder space doubles and the faster still the powder has to make gas if pressure is to be maintained or increased.
That brings us to sectional density. This is the other thing that affects how quickly the powder space can grow. Sectional density represents how much bullet mass each square inch of the bullet base has in front of it. Since the force applied to the bullet base to move it forward is the pressure divided by the number of those square inches of base, the less weight each square inch has in front of it, the less inertia that force has to overcome and the faster the bullet accelerates under that pressure.
So, if you have a bullet with a short powder space, and a wide bullet with low sectional density, it doesn't take much pressure to get the bullet scooting forward pretty fast, and you need a fast powder to make gas fast enough to keep up with that. Otherwise you have to settle for just burning powder real fast and accepting that pressure will drop off real fast as the bullet moves forward, expanding the powder space.
Both strategies are used, depending on whether the powder you use is digressive of progressive. Digressive powders like Bullseye and the very fast Vihtavuorin N310 or Hodgdon Clays, burn from the outside surface in, and the material burns equally fast until the grain is consumed. So it makes its gas as fast as possible and quits, and when that's done the pressure in the bore drops as the bullet moves forward. These powders are called digressive because, if you burn them in a special constant pressure vessel, the rate at which they make gas diminishes as the grain burns inward. That's because the burning surface area is getting smaller and smaller and the rate at which the burn eats the grain inward is constant in constant pressure.
Progressive burning powders do just the opposite. The longer they burn, the faster they make gas until they run out. There are a couple of strategies used to make this happen. With stick powders they put little tunnels called perforations inside each grain, then put a deterrent coating on the outside of the grain so the outside burns very slowly. But not the inside. So the flame spreads into the perforations where they burn from the inside out. That means the diameter of each tunnel keeps getting bigger, providing a bigger and bigger burning surface area that provides gas faster and faster until it runs out of tunnel wall material.
Spherical progressive powders have deterrent coatings that penetrate into their surface, becoming less concentrated as you go from the surface toward the center of the grain. This allows the outside surface to begin burning slowly, but speed up as it burns toward the center. In that way they can make gas faster and faster up to the point the remaining surface area is so small that even burning at its top rate it can no longer keep it up.
The idea behind the progressive powders is that because they start burning slowly, you can put a lot more powder in without making enough gas to created too much pressure when the bullet is just starting to move. As the bullet goes down the tube a progressive powder makes gas faster and faster, actually raising the pressure as the bullet moves and expands the space. It continues that way until it burns the bulk of the powder out. The slow burn start gives the bullet time to expand the chamber before a progressive powder has made all its gas. This means the pressure peaks in a larger total powder space, which takes more total gas than reaching the same pressure in a smaller space does. So, now as the bullet goes still further forward after the pressure peak, the pressure drops off less rapidly than it would if the pressure peak had been reached with less gas in a smaller space. In effect, by delaying the peak, progressive powders get more total space to peak in which lets a larger quantity of the powder be safely fired . This gives you more gas which keeps acceleration from dropping as quickly as the bullet moves beyond the the peak pressure point in the bore.
In real life it isn't quite that simple, but that's the general idea. Even a big charge of progressive powder in an overbore rifle can't keep up with bullet expansion for more than a few inches before pressure starts to drop. There are several reasons. One is just that the bullet has picked up around half its velocity by then, so it is running away and expanding the powder space really fast from that point forward, making it impossible for gas generation to keep up. Also, once the bullet is going really fast, it starts to challenge the speed with which the gas can flow forward after it, so a partial pressure drop develops between the chamber and the bullet base. Another factor is that the powder doesn't actually light up all at once. It normally starts at the flash hole and the flame front propagates forward, so the powder just behind the bullet gets lit later than the rest. This is why you find some unburned grains on the ground out in front of the firing line. The larger and slower you make the charge, the more of that you get, so there's a point where going further just makes for more unburned powder and the bullet doesn't go any faster or has actually lost velocity as compared to a faster burning powder charge. Such slow powder requires higher start pressures to help burn through deterrent coatings, and has a harder time achieving them. As a result, ignition becomes erratic and velocity variation increases.
"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity."
Homer Powley
