Temp changes?

[KeithR41]

Here's the scenario, I set up my target at 1408 YDS, 4480 feet elevation, 90 Deg. temp. I'm shooting a 300SMK at 2880. FTE gave me 43 moa of elevation.

92.5 grn of H1000. And the ammo had been sitting in the sun all day.

When I shot I had to adjust to 35 moa. Normally FTE is spot on but this wasn't even close.

So......I played with the numbers of FTE and found that just over 3000fps gave me 35 moa.

How much will tempature change in the loaded round change the velocity. Also, didn't have any pressure signs.

Thanks, KeithR41

 

[Lowlight]

Re: Temp changes?

How were inputting the Barometric pressure and altitude...

Just off the cuff, JBM gives me 37MOA - I can move the MV to 2950 and get 35MOA which might not be a stretch being so hot if the powder is a bit temp sensitive and your rounds were in the sun getting hot, etc, ambient temp versus direct sunlight.

 

[Lowlight]

Re: Temp changes?

Also, you weren't happening to be shooting at an Angle where you ?

 

[KeithR41]

Re: Temp changes?

Frank, very little angle. There was about 10 feet of difference between the target and myself.

I didn't mess with barometric pressure and I just input the temp into FTE manually.

KeithR41

 

[Lowlight]

Re: Temp changes?

So your Baro was set for 29.92 and what was your altitude set at ? and did you have it turned on to be corrected.

You can't just use Temp, you have to use it all for it too work.

 

[dareposte]

Re: Temp changes?

I've noticed with H1000 the powder is a bit temp sensitive, so your MV may have been up just from having the ammo laying in the sun all day. It't not as bad as my IMR4064 is but it will change 50fps no trouble from being in the 80's and shade to being mid 90's and in the sun.

I'm not familiar with FTE in particular, but checking your data it looks like you might have had it calculated for sea level instead of your altitude. www.dop3.com gave me 42.7 MOA elevation at sea level, 90F, 29.53inHg. Switching the altitude to 4480ft gave me 36.8 MOA, so thats within the realm of possibility if you didn't correct for station pressure and humidity, and maybe had an increased MV due to ammo heating up in the sun.

Really 1400yds is kind of outside the accurate range of ballistic calculators anyway, they'll give you an estimate but thats a long way and a lot of variables to nail down. Without radar range data and specific drag models the bullets just don't behave like the computer claims they should.

 

[KeithR41]

Re: Temp changes?

Frank not sure what you mean by "turned on to be corrected." I did have the altitude set to 4480 and the barometric pressure was whatever the default value is, I believe it is 29.92. I don't have a Kestrel or any other way to check that.

 

[Lowlight]

Re: Temp changes?

Read this stuff, it will help you, because that is your problem.

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Baro/

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/

 

[KeithR41]

Re: Temp changes?

Figured out what I was doing wrong. I was
changing the zero atmosphere instead of the
current atmosphere.

Thanks for the links. I'll read them tonight.

Did I mention that I've neverbeen tested but I
might be legally retarded!

Keithr41

 

[Augustis]

Re: Temp changes?

Keith
My advice, set the altitude to "0" & input station pressure(actual air pressure) only. For example, I live at an altitude of roughly 4400 ASL, presently the BP - AP or station pressure reading is 25.43 in/hg. The difference between BP & SP is, the SP is not standardized to sea level, meaning the same BP reading the news would disclose at this moment would be 29.43 in/hg. Station Pressure, or Air Pressure is the actual air pressure at your AO. IMO this is the ticket.

Aug ><<>