Weather & air density effects

[Mike41]

Does a high air density (100.4) along with cool temps (42 F), low barometer pressure 29" effect power factor? Or does the air density reduce the veloicity?

Seems like my loads had a lower than expected PF? I am thinking the air density is the biggest effect.

Also how will OAL effect velocity? Shorter is faster if same bullet, & powder?

I trying to understand my data and how it all relates.

Thanks,

Mike

[jcc7x7]

I've found OAL doesn't effect velocity a whole bunch if in the realm of reason BUT

it does effect accuracy and ES and SD

I've shot some of the same loads from the Texas panhandle to upstate NY and a bunch

of places in between and only had about 1 or 2 PF changes.

that's the reason I load to 172 or 173 for limited

140 in minor cause it runs the gun better

so there is no problems regardless of altitude or temp.

[MarkCO]

With some pistol powders, low temperature can reduce or increase pressure affecting velocity. Increase air density or water content and velocity is slightly reduced. The worst I have ever actually seen was 4 points in PF, which was less than 25 fps. Some people have SDs larger than that in their anmo.

[PKT1106]

I don't think it is air density at 10ft in front of the muzzle. Temp of powder is more of a factor.

Air density and humidity is more more longer range rifle ballistics.

[Nevadazielmeister]

I don't think it is air density at 10ft in front of the muzzle. Temp of powder is more of a factor.

Air density and humidity is more more longer range rifle ballistics.

I agree completely.

Muzzle velocity is dictated by many factors, but the variation between shots in different atmospherics is solely due to powder temperature. If you have ever been to a long range rifle match, some participants keep their ammo in a cooler. I use a Applied Ballistics Kestrel which calculates my muzzle velocity based on the temperature, which is pretty much a linear graph.

At 80 degrees Fahrenheit I get 2901 fps for my .308 and 2840 fps at 40 degrees. I suspect that the difference in muzzle velocity is due solely to the pistol powder, which is more affected by the temperatures than rifle powders (especially Varget).

[jstagn]

Some powders like E-3 get faster with lowers temp most get slower like Clays, probably not enough to worry about unless you are going to a major match where they chrono. SO chrono as close to the match as you can with the same expected temps and add .1 grain so there will not be any surprises at the chrono. Different chronos can read slightly different. Keep everything the same, primers, bullets, OAL and crimp etc.....

[Mike41]

Here is my data for 5 shot strings.

All from the same setup with no changes to my 550b & shot back to back using STI edge.

Conditions: cloudy, 42.3F, 62% RH, 39.16" barometer pressure, air density 100.4

Reloads from a month ago 5 shot ave.

OAL 1.187 fps ave. 715 pf 128.8

180 gr zero JHP using N320

Reloaded 12-27-14

OAL 1.183 fps ave. 782 pf 140.8

180 gr zero JHP using 4.1 grs n320

OAL 1.184 fps ave. 855 pf 153.97

180 gr BBI Flat nose using 4.1 grs n320

OAL 1.126 fps ave. 677 pf 121

180 gr Atlanta Arms TCI minor

Not sure why OAL decreased from a month ago nor can I understand why the month older reloads had a refuced pf. Again I made no adjustments to the dillion machine except add primers. Would the Dillion machine vary that much in powder dispense? I thought a month ago I was actually getting 4.3 gr. but since I was going to chrono these rounds I recali rated my beam scale with the rcbs weights and measured each of the rounds. I did not change the powder adjustment however.

I am also surprised the Atlanta Arms were below 125. I was expecting 130 or so.

So, everything thing seems about 7~10 pf lower than I was expecting. Rounds and powder were inside until I shot with the BBI being the last rounds fired and the old reloads the first. Would in the course of 20 minutes from 1st shot to last shot vary that much due to temp? I am just saying as the older month reloads were shot first and the BBI last And the power factor seemed to increase with each group shot.

If I repeated this again in 85F high humidity, high barometric pressure and lower air density of 89% what would I expect to see?

Thanks & Happy New Year!

Mike

[zzt]

I haven't found e3 to be reverse temp sensitive. In fact, I've not found it to be temp sensitive at all. I just looked at my chrono data, and i got exactly the same fps and PF at 85 as I did at 39. I'll chrono some of the same batch in the middle of winter and see if there is a change. I'd bet not.

At pistol distances, air density and humidity play almost no role in velocity drop. As stated above, it is the temp sensitivity of the powder that causes the change. For example, one of my fav 45 ACP loads is 3.8gr N310 with a 200gr LSWC. That's was my bullseye load ( I currently use 4.0gr). At 85 deg it produces 749fps. At 39 it goes 674. I didn't chrono at 24 deg, but the brass just dribbled out. It is the most temp sensitive powder I've encountered so far.

[jstagn]

That's are lot of variation with n310, maybe because its a very fast powder. Wonder how n320 works since I intend to use that in 40. Thanks for the examples...

[Mike41]

Maybe VV powders vary a lot with temp? As I am using VV N320. I wonder what Atlanta Arms is using? Does anybody have any chrono data on the Atlanta Arms .40 minor out of a STI edge besides my data above? Is it normally a very low pf load?

Mike