CR,
A magnum primer might help, too, especially in cold weather, though you want to start the load work-up over again if you try it. Look for decreased velocity SD as proof the primer is being helpful. Also, make sure you seat your primers firmly:
"There is some debate about how deeply primers should be seated. I don’t pretend to have all the answers about this, but I have experimented with seating primers to different depths and seeing what happens on the chronograph and target paper, and so far I’ve obtained my best results seating them hard, pushing them in past the point where the anvil can be felt hitting the bottom of the pocket. Doing this, I can almost always get velocity standard deviations of less than 10 feet per second, even with magnum cartridges and long-bodied standards on the ’06 case, and I haven’t been able to accomplish that seating primers to lesser depths."
Dan Hackett
Precision Shooting Reloading Guide, Precision Shooting Inc., Pub. (R.I.P.), Manchester, CT, 1995, p. 271.
Here's what I get for your data. Because you have only 3 samples where I figured the SD, I used a
low sample bias elimination method of calculating it, and these numbers are about 13% bigger than you will get if you use the standard SD formula which is what your chronograph will use, but are more likely to be accurate.
What you can see is the standard deviation is really high at 41 grains but shrinks as you get toward 42 grains. The velocity seems fairly level for the last four points, but with the high standard deviations, you can't really be sure that's not random happenstance. The bottom line is you would need to shoot more rounds to narrow the error.
So, what is causing such a wide deviation? Your suggestion of irregular ignition is one candidate. The chronograph not handling the temperatures is another. While cell phones and computers and more complex automotive electronics have changed it some, a lot of consumer-grade electronics still aren't rated to operate below freezing. When they get there or too close, errors can occur.
Another factor can be batteries. Alkaline and carbon-zinc batteries lose a lot of their output ability in the cold.
Lithium batteries do much better, and I recommend those for cold weather.
Finally, you can check your chronograph roughly with a 22 rimfire rifle. Because 22 rimfire has such a huge expansion ratio, all its powder is gone by the time a bullet has gone 20" or more down a barrel. Beyond that, the bullet is slowing down in the bore, albeit very gradually. This fact eliminates a lot of the effect of chamber variation. So if you buy match-grade 22 rimfire ammunition and have a rifle with a 20"-26" barrel, you will find it shoots within 50 fps or better of the velocity claimed by the manufacturer for the round. So if you shoot ten of the match rounds over your chronograph, it should behave quite well and the reading should be within that 50 fps limit if the chronograph is working as it should.
Another approach: If you know someone who will loan you a second chronograph, you can shoot over both of them in series, one right behind the other, and make sure they agree pretty well.
