What you're measuring is the "throat," or leade. The chamber area forward of the bullet has a section of bore-diameter barrel that has no rifiling. The rifling lands have ramps to ease the bullet into the rifling. The cylindrical section is there to reduce pressure. Specifically, to handle the higher pressure of 5.56 vs. .223, and to deal with longer or harder bullets. The M855 is longer than an M193, and the tracer round is longer still. By having the "jump" the chamber pressure stays where it is supposed to be even with longer bullets, hot or dirty chambers, etc.
Benchrest shooters and long-range target shooters like to load long to minimise the jump. It does improve accuracy (to a small but measurable degree) but it does so at the expense of pressure restrictions. Such loads have to be tailored to the rifle they are used in, and may be unsafe in other rifles.
If the manufacturers were the stretch the upper, lower, magazine and carrier to close the gap you see, barrel makers would simply extend the leade to make sure there was a safety margin.
Load to magazine length, with published data, and leave the fussing to the 600-yard shooters.