I've been unable to find much about chronograph outliers online, probably because I don't know how to word the search.

Equipment:
Kimber Gold Combat II (5" 1911)
45 ACP (various loads)
Shooting Chrony F1 Master

I was shooting about 8 feet back. I won't post the specific load, let's just say it was a 230gr XTP over Universal, near max for standard pressure 45 ACP.

Most of the readings were normal, between 820 and 880 feet per second. But, I started seeing some extremely low numbers, like in the 500s and low 600s. Not just one or two, but 7 out of 25 fired were below 650 feet per second.

I saw 3 of 25 under 600 fps in the next load I tested, 230gr XTP over WSF, under max, where a typical reading should be in the low 800s.

To complicate diagnosis, my recoil spring was dying. I replaced it after the WSF load, and my next string (230gr XTP over WSF, max load) showed no strange readings except one that was <300 fps, which must have been a chronograph error since it cycled the pistol like normal with a new 18.5# spring.

So, my only real theory is that the dead recoil spring was letting the pistol unlock prematurely and pressure was falling off, resulting in the low readings. Another bit of evidence there is the firing pin marks were "smeared" sort of, like the firing pin struck and the barrel unlocked while the firing pin was still in contact, dragging the pin on the primer. The firing pin is fine; I checked it for breakage or deformity, but I did replace the firing pin spring for the heck of it. I've attached a picture of the firing pin strikes, the two on the left are the shady ones, the two on the right are normal strikes from the same group.

With respect to my loading practices, I use an RCBS uniflow measure and check weights with the 5-0-5 scale. I typically check every tenth charge for consistency, and visually inspect the cases before seating bullets. I'm confident my charge weights were accurate. I use a Lee factory crimp die and have never seen setback in any of the thousands of XTP loads I've made, and I don't see how setback would result in a lower velocity anyway.

I'm pretty sure I've diagnosed the problem correctly, but if anyone has any experiences or thoughts to add, I'd be interested in hearing them.

Yeah, that's what prompted me to replace the firing pin spring. I'm pretty sure the old firing pin spring had seen several thousand rounds, plus who knows how many dry fires--twenty thousand? More? It's been in the gun for at least two years, as my daily carry / main range 1911. It did seem pretty soft compared to the new one when I took it out. Might even be original... I can't say I've had to replace firing pin springs too often on my various 1911s.

I'm also curious why the strikes on the left seem a bit lighter than on the right. They're the same primers, CCI 300s, but in the various shady looking firing pin strikes, they are noticeably (if not drastically) shallower dimples. Maybe it's time to replace the mainspring as well. Can't hurt, I guess, but I also don't think it would make any meaningful difference in the velocity as long as the primer fired, so that's not really applicable here.

It was a brand new Duracell right out of the package. With the 500-600 FPS loads, I could definitely feel the difference, since I was firing one careful shot at a time instead of running drills like normal. Even with the dying recoil spring the brass from the slow loads was falling 3-4 feet away, while the loads that read full speed were launching themselves into the grass 12-15 feet away. There was a clear difference in power. The lowest velocity recorded, 523.3 feet per second, resulted in a failure to feed.

Just checked the voltage on the 9V I was using. 9.19V, so that's not it.

Other than that one odd <300fps reading, the chrony was good to go the rest of the day after I replaced my recoil spring. I shot another three 45 ACP test loads, 25 rounds each, over it with good results, and fifty or so 308 Winchester loads through my M1A and LR-308.

Thanks for the input.

I've got an easy way to test it, I guess. I'll load up 100 rounds of the specific load that was so shady, and run half over the chrony with the old spring and half with the new. Primers from the same 1000 primer box as the last set, powder from the same can, everything exactly the same (as much as possible) except the spring. I'll accept a little frame battering to figure this out. I'm glad I didn't clean out the range bag yet or I wouldn't have the old spring anymore.

I'll be back in 5 hours or so with results.

Also, I don't mean the pistol was unlocking per se, since my cases look fine. But maybe the difference in slide velocity would change the way the powder burns? Who knows. Buffalo Bore suggests that's possible, with respect to high extreme spreads in their heavy 10mm loads, but not to this extent. That's where I got the idea to begin with.

I had another thought on this.

Is it possible some of your bullets are pulling as the cartridge is chambered?  This would cause more case capacity and less pressure behind the bullet, thereby reducing velocity.

That would be almost impossible... bullets are much much more likely to get pushed deeper into the case.

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.

I'm not sure if I can be of much help here, but heres something to think about also....the 45ACP casing has a usable powder capacity of 1.14cc....if the charge of the powder that you're using doesn't fill the cavity under the bullet enough, and the round is being shot on a level plane or a downhill plane, then the primer strike will only burn off the top of the powder first giving an erratic reading on a chronograph and erratic velocity with different pressure readings...I had this problem using Bullseye powder that only partially filled the casing, but by pointing the pistol up and then levelling it off on target, I got consistent readings on my chronograph.
I got less troublesome reloads using a bulkier powder and got to calculating the airgap (space between the top of the powder and the bottom of the bullet)...and try to keep that feature under a quarter of an inch.
This may not be the problem, but is something to think about on any reload.