[carguychris]

The optimum burn rate depends on the case capacity, bullet weight and shape, and the barrel length. In very simplistic terms, slower powders generally require longer barrels, to give the expanding gases more time to work.

FWIW "burn rate" is actually somewhat of a simplistic misnomer; the powder normally burns more or less instantaneously, the difference is the pressure curve created by the expanding gases. The pressure curve of a "slower" powder peaks later.

A real-world illustration is firing milsurp ammo intended for a full-size rifle or machine gun in a short-barreled carbine; this often results in very pronounced muzzle flash and blast as the gases are prematurely vented to the atmosphere.

As the OP points out, burn rate can be a concern in some semi-automatic rifles, particularly older and less flexible gas-operated designs such as the M1 Garand. However, as Mike points out, the majority of commercial ammo is perfectly safe for most firearms.

One of the major attractions of handloading is the ability to "tune" the load to the gun, particularly if it has a shorter or longer barrel than usual for the caliber.