I'm not worried about that 300 fps reading, it was just an odd fluke, and wasn't repeated, unlike the other outliers.
The sun is usually at such an angle that it doesn't affect the chrony, and I tilt it back away from the sun slightly anyway simply because I usually have the chrony on a tripod on my pickup tailgate and I'm shooting into a hillside at a downward angle.
At any rate, I loaded 100 rounds to the same specs as the previous Universal load that showed 7 of 25 under 700 feet per second. I shot 50 with the old recoil spring, and 50 with the fresh one.
Old spring average velocity, after throwing out outliers, was about 790 feet per second. I had three under 600 feet per second, similar in recoil and showing the odd smeared firing pin strike like before.
New spring average velocity was 810 feet per second. Slowest recorded bullet was ~740 feet per second, everything else was within one standard deviation of the average (SD 31 fps).
The velocity distribution was much broader with the old recoil spring in, ignoring the outliers. I was very careful loading these rounds. The only significant difference between the two runs was the recoil spring.
Of course, given the poor distribution in general, I can conclude that this load kinda sucks. But that's not the point. It sucked a lot more with the worn out recoil spring than with the fresh one, all else being as equal as I could make it.
I'm having a hard time buying that my chronograph would be giving me truly funny readings only when shooting a 1911 with a worn out recoil spring. I mean, I don't just test these 45 loads. I'm working up loads for my M1A, LR-308, and various 41 and 44 magnum loads for bear defense (I live in Alaska) and any odd readings I get are ridiculously high, from muzzle blast, never super low (yeah yeah, 300 fps, who knows what happened there, it didn't happen again is the point). So each time I go out with the chronograph, I might shoot three hundred or more rounds over it. If it was an environmental condition, or a bad chronograph, or a low battery, surely I would be seeing other odd results. I've fired every gun I have over it, and the only repeatable strange stuff I've seen was in the specific circumstances I tested here.
I'm convinced that the recoil spring is the significant factor here. I'd like to do more testing, but I'm not going to waste a lot of components and add unnecessary wear to my pistol for no good reason other than scientific rigor. This just tells me that I need to actually start keeping a logbook for each of my pistols and replace the springs at predetermined intervals to avoid such problems in the future, and it's an interesting experience that adds to my troubleshooting repertoire.
Norseman, thanks for the heads up on bullet sticking. I always observe the bullet strike, and I certainly would have been kinda worried if I didn't see one (and would have checked the barrel, assuming I loaded a squib or something), but it didn't even occur to me to worry about it. I've never really loaded on the slow end of things, so all I know about reduced loads is too little powder can detonate rather than burn.