Temp differences and MV

@Unclenick, load was the rough load development at 100 yards back in mid December in mid 70's. Last Monday or Tuesday I did a sight in at 600 when it was in mid 40's then the match yesterday started at 8 am at around 30F increasing to low 40's by afternoon. I wasn't expecting much from the load but ended up at a solid 96% with only one flyer in the 8 ring that was my fault for shifting my weight with my finger on the trigger otherwise it was a pretty solid load considering the development, some mild wind conditions and mirage. Yes Virginia you can have mirage in cold weather. I looked through a Celestron sighting scope set at 60 power and it looked like a pinwheel, at 24 power through my rifle scope it was milder but visable

here is the thread for the original load workup back in December

https://thefiringline.com/forums/showthread.php?t=599693
 
The range at which the drop is measured can matter significantly, remembering bullets travel in an arc trajectory. At 600 yds, likely the bullet is already dropping. More of a drop may be related to a decrease in velocity. One of the common trajectory (come up) programs is very useful for checking this. If the increased "drop" is at an earlier part of the trajectory where the bullet is still climbing or level, the increased "drop" may be indicative of an increase in velocity.

Most of my rifles are sighted in at 100 yds. If the bullet starts impacting noticeably lower at 100 yds, one of the first things (and there are others) that may be occurring is an increase of pressure / velocity. As in the bullet may be leaving the barrel earlier in it's rising recoil.

Course it is just an opinion of what i would be checking for my own purposes.
 
I would have thought that the drop in POI that hounddawg experienced indicates a drop in pressure and velocity not an increase.
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yep it was a decrease

Based upon the Hodgdon data on H4350, I wouldn't have expected a significant a velocity change. If it was IMR4350 I would have expected it
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that's what I would have thought also. If you look at the original load workup there was an increase of about 6 to 20 FPS every three tenths of a grain, but the numbers are what they are. In retrospect I should have bumped my charge up a tenth or two, I had decent groups as far as my CEP numbers went but the groups were not as tight as I would have liked and my X count was low. I think a load bump up would have helped that

Like I say I am just learning this stuff and trying to figure it out myself. It helps throwing it out here and getting opinions on the why's
 
zeke,
I didn't think about the bullet leaving the barrel earlier in the recoil cycle if it was at a higher velocity.
That could explain the rise in PIO.
 
I have long suffered from inconsistent and wandering groups with my hunting rifle though.
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A lot of hunting rifles aren't made to shoot good groups. They're made to put one, or two, possibly 3 shots at or close to the point of aim, and after that, not so much. If shots 3-5 are within a few inches of 1-2, that's usually minute of deer, and good enough under usual field conditions.

Varmint and match rifles are much different creatures than the usual deer rifles.

There are a couple of things I didn't see touched on enough talking about the idea of shooting chilled ammo to replicate cold hunting conditions on a warm day. First, one that was mentioned is wearing those same cold weather clothes or not. Wearing your cold weather hunting gear vs. a t-shirt at 79F DOES affect your shooting.
Another point, is the temp of the RIFLE. Everything behaves just a little differently in the cold than the heat. Some things behave a lot differently.

20F is enough below freezing to qualify as cold, in my book, but its not the same as sub-zero temps, and I have hunted in both. Likewise, when its 110F in the shade and there is no shade, things act differently.

Long standing rule is to work up your top loads in the summer, and you will be safe in the winter. The reverse may not be true.

A one of things I learned as a youth in deer camp in the middle of an Adirondack winter was that when there was a nice warm cabin at the end of the day, the guys that knew what they were doing left their rifles and ammo outside!
 
My season for shooting is in the warmer months 50 degrees on up , I'm only shooting 200 yards my rounds vary within 2" in drop only from 50 on up . Cold being 50 the lowest group as it reaches the 70's shooting at a 1" square I don't adjust just horizontal anymore just hold at bottom middle and top of the 1" square target as temps fluctuate. Using IMR 4064 mild load 168 gr Sierra MK
 
Someone above mentioned that some powders vary in burn rate more than others ..... my experience is that slowish ball powders, like H414, are harder to ignite evenly in cold weather ....... SD as measured on the chronograph grows and MV drops ....... substituting a magnum primer for the standard primer solved the issue for me at the time..... though I never did try that load in the summer heat ...... probably not a good idea.
 
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