Normal gun propellants for large bore guns are produced in propellant grains. These grains are fairly good size, measuring about half an inch in diameter and about an inch or so in length.
Consider a solid propellant grain and the way it burns. Make the assumption that any particular grain will begin to burn everywhere on its exterior surface simultaneously. Overall burn rate is a function of the particular propellant, breech pressure, and the exposed surface area that is burning. As the propellant begins to burn, the pressure behind the projectile continues to rise until the projectile begins to move. When the projectile begins to move, the volume in the breech increases.
When the volume increases, the pressure in the breech tends to rise slower. the point is that projectile motion
will adversely affect breech pressure because breech volume keeps
increasing. This, in turn, causes a drop in burn rate, or at
least adversely affects the acceleration of burn rate.
At the same time as this is going on, the surface area of
the propellant grain is also decreasing because it is being burned away, exposing more propellant, which also burns away, making it even smaller, ... ad infinauseum. The net result is that propellant gas is generated at much slower rates than you'd like and the projectile doesn't come out as fast as you'd like.
What you really want to have happen is to have the pressure behind the
projectile rise as continuously and as smoothly as it can.
One way to help do this is to use what is called a
perforated propellant grain. These things look like strange
hamburger helper noodles. Instead of a single hole, or
"perforation", through the center, they have a bunch. Seven- and
nine-perf propellants have been used. The idea here is that when
they begin to burn all over, "all over" includes the insides of
the perforations too. So, while the outside burning surface is
getting smaller, the inside burning surface is getting bigger. If
the propellant type, grain design, gun, and projectile are all
properly matched you get a smoothly rising pressure as the
projectile moves down the barrel, but not for the entire length
of the barrel
Now, consider the size and shape of pistol/rifle powder.
What do you think would happen if you were to use it as the
propellant for a large bore gun? It has a LARGE total initial
area to set on fire. The initial burn rate would be so high it would cause the pressure to spike behind the projectile before it can move, which would raise the burn rate, which would spike the pressure further, which would.....BANG!!!!
Solid propellants are just that - solid. You can predict what the
surface area exposed to burning is as a function of time with
them. You can't do this as well with liquid propellants because
they slosh around and change their exposed surface shapes and
amounts - even while burning. Also, one of the biggest
variables in gun accuracy at range is muzzle velocity. They will
need a VERY predictable burning rate in order to properly control