Typical nitro "target" loads have a pressure curve that quickly reaches a higher peak pressure and then taper off very quickly thereafter. If you imagine the pressures in the barrel while also imagining the position of the shot charge as it travels down the barrel, with nitro the peak pressure is achieved while the shot charge is still nearer to the breech. After that point, the shot charge is actually decelerating as it continues down the barrel and down range.
The nitro "low pressure" loads use blended powders that are 'retarded' so that the powder burns more slowly (but still burns completely before the shot column reaches the muzzle). The result is the same relative velocity by the end of the barrel, while generating lower peak pressure.
The science that helps this make sense is as follows:
The amount of nitro powder for both kinds of shells is similar. The significant factor is the amount of gas produced by both loads (this will be very close to equal). The powder in the "target load" releases it's energy VERY quickly, producing it's total volume of gas while the volume of barrel space is still very small. The pressure produced is higher because the same amount of gas is produced in a smaller space. After that, the pressure is actually decreasing rapidly as the shot charge is pushed down the barrel.
The "low pressure" loads are charged with enough slower-burning powder to produce the same amount of gas. But, due to the slower burn rate, the full amount of gas isn't released until the shot charge is just about to exit the muzzle.
Since both loads produce the same amount of gas, the net gas expansion behind the shot charge is almost identical by the time the shot leaves the gun which, in turn, produces similar muzzle velocities. The major difference is how fast the gas is produced, and this is what determines the peak pressure.
