here's a good one. so i bought my second automag not too long ago (older 68 automag air valve). aired it up and chronoed it, and it wouldn't go higher than 250 fps. so i tore it apart, thoroughly cleaned the regulator, installed a new piston o-ring and reg seat, put fresh oil / grease / anti-seize on the relevant parts. i tested it on my working automag (newer 68 automag classic) and it would chrono at 280 fps no problem. so back onto the older automag, and...250 fps.
after swapping various parts around, i determined that the bolt was the issue. for whatever reason, using the old bolt resulted in a low velocity, while the newer bolt had no issues. so, i guess it was old and worn out? before i toss it, i get out the caliper just to compare measurements, and i hold it up to the light and notice the holes that the air pass through are much smaller on the old bolt than they are on the new bolt! also, that light could pass through a part inside the bolt.
that's weird. i look closer and...there was an old regulator piston o-ring shoved inside the bolt! if that wasn't enough, there was an old power tube o-ring pushed all the way down the stem on my newer bolt. i pushed the o-rings out by shoving three tootsie-pop popsicle sticks down in there (they're paper, so it wouldn't scratch anything) until they came out of the back. i re-tested the bolt and there are no more velocity issues.
so yeah, if you have low velocity problems on a classic valve, and you know it's not the regulator, check your bolt and make sure there's nothing weird inside there!
as an aside, while i was overhauling the regulator, i noted a buna o-ring on the regulator piston (which is of course, not correct). i guess i found out where original piston o-ring disappeared to! LOL!