Terry, some of my 5.56 loads are at the point where they're almost compressed loads. Especially the heavies. No crimp, no cannelure projos, and one of those catches going out of the mag and into the chamber - bullet setback, which definitely compresses the load.
Pressure spike, right there, and it's in the chamber, bolt closed on it. You never get a chance to see it, during the loading process. Pull the trigger on that one, and the rifle can come apart in your hands.
Not a big deal on a bolt gun, but it's a big deal on the autoloaders. There's some hellish spring pressure shoving a heavy bolt carrier to load that next round. One cartridge with a setback projectile can do much more than ruin your day, if it causes that rifle to come apart.
APR, projos with a cannelure are better for the autoloaders, and hold a crimp better, but they're not necessary.