mikld, Perhaps you were thinking about peak chamber pressure, which does indeed happen in the first few tenths of an inch of bullet travel ... however, powder takes a lot longer to totally burn up. Just for grins, I fired up QuickLOAD and took a look at some popular powders in a 357 Mag. Other magnum cartridges follow a similar pattern. All listed loads are under the SAAMI max of 35k psi. I used 140 gr bullet max loads from a Speer #14 manual then plotted bullet travel versus % of burn. Here's the results in order of fastest to slowest burn rate:
W-231, 7.1 gr, 95% powder burn @ .85" of bullet travel; 100% burn @ 1.49" of bullet travel. Velocity with a 4 5/8" barrel = 1133 fps, minimal muzzle flash
Unique, 8.0 gr, 95% powder burn @ 1.3" of bullet travel; 100% burn @ 2.5" of bullet travel. Velocity with a 4 5/8" barrel = 1233 fps, minimal muzzle flash.
Power Pistol, 9.5 gr, 95% powder burn @ 4.4" of bullet travel; 100% burn @ 13.9" of bullet travel. Velocity with a 4 5/8" barrel = 1265 fps, modest muzzle flash.
2400, 15.1 gr, 95% powder burn @ 18.6" of bullet travel; 100% burn off the charts. Velocity with a 4 5/8" barrel = 1398 fps, heavy muzzle flash.
W-296, 18.0 gr, 95% powder burn @ 23.9" of bullet travel; 100% burn off the charts. Velocity with a 4 5/8" barrel = 1326 fps, very heavy muzzle flash.
As you can see, any powder slower than Power Pistol is going to create a notable muzzle flash ... the slower the powder burns, the greater the flash. Also with slower burning powder, velocity continues to increase considerably with longer barrels whereas with faster burning powders, velocity increase is a token amount.