Gun: Colt Officers Model 38 (so shoot me )
Bullet: Rainier plated 158 grain flat point
Cases: UMC factory ammo, third reload
Primer: Federal small pistol
COAL: 1.455"
Powder: Win 231

Rainier says to use lead bullet data but my velocities didn't come close to any lead bullet data I found. I built a ladder from 3.6 grains up to 4.4 grains based on available data. At 3.6 grains, they were barely dribbling out of the barrel and I feared the last one was a squib (chrono showed it at 130 fps). Extreme spread and standard deviation data were a mess, along with the "groups", until I got up to 4.4 grains:

avg 670 fps
hi/lo 62
std dev 25

So accuracy was terrible and my velocities were way below lead data from any source. My Hornady manual shows 4.4 grains as the MAX for a 158 grain lead bullet. It does show 158 grain hollow points over 4.3 grains at 700 fps. That looks a lot closer to my results. For the jacketed bullets, their MAX is 4.9 grains at 900 fps but that load is rated +P. Sierra shows 4.9 at 850 fps as max for jacketed 158 grain bullet.

I don't plan to go all the way up to 4.9 grains, but do you all think working up to 4.7 grains in 0.1 grain steps is responsible?

As a side note, this revolver loves 3.5 grains of W-231 under a swaged 158 grain Hornady HBWC. Avg 834 fps; ES 36; SD 9; and a 5-shot group under an inch at 15 yards off rest. There's nothing wrong with the gun.