bludog,
I didn't detail any of the possible reasons for your low velocities, so will now.
Many other people have reported RL-16 velocities similar to those in Alliant's data, and in fact you're the first person I've read about who reported low velocities. Several possibilities exist for this to occur:
First, the assumption your chronograph was working correctly. Among around a dozen chronographs, I've owned two of one particular brand of popular chronograph that eventually proved unreliable, and seen the same brand causing trouble for other shooters at local ranges. My second one recorded velocities 100 fps difference depending on whether the sky was clear or cloudy. Low batteries can also cause problems. We don't know what chronograph you used, or if the RL-16 loads were the only ones you tried that day. But if you chronographed other loads during the same session and they all produced "normal," consistent velocities, we can rule out chronograph aberrations.
You happened to get a slow batch of powder. This can happen, but isn't common, and when it does velocity difference is rarely 200 fps. In fact I don't recall ever seeing as much as 100 fps.
You used a different bullet than anything listed in Alliant's data, and these days the pressures (and therefore velocities) produced by different bullets vary considerably. In fact different brass and primers can also result in lower velocities, as can seating bullets farther off the lands. I've seen as much as 250 fps difference in velocity when using different bullets of the same weight, with the same powder charge, in the same rifle.
