Question on DOPE Discrepancies

[richthe1]

I’m new and still trying to learn the ropes. Why did I have such a large discrepancy between my first and second outing? Are these results normal? Is FGMM that temp sensitive?

Ammo: 308 FGMM, 175gr. Note: different lot # between Outing 1 and Outing 2.

OUTING 1 (shot over 2 consecutive days, roughly 300rds total)
Temp=between 80F-97F
Altitude=1100ft
Rel. Humidity= between 40-65%
Baro Press.=between 29-30
Wind: 5-10mph from 10-2 o’clock

100=0
400=2.2
500=3.2
600=4.2
700=5.6
800=6.9



OUTING 2 (Note: I cleaned well before Outing 2. Roughly 70rds fired during Outing 2)
Temp=48.6F
Altitude=610ft
Rel. Humidity=60%
Baro Press.=29.41
Wind=10-15mph from 7-8 o’clock

100=0.1
400=2.6
500=3.5
600=4.8
700=6.3
800=7.9

 

[Deleted member 113831]

There shouldn't have been anywhere near that much difference at 800. But optical effects can and will do that. Without being there to see the terrain and lighting conditions no one can really say. Shoot it again in overcast or early in the morning and see if there is a difference. That will tell the tale.

 

[Blackjack988]

I'll let someone more knowledgeable correct me if i'm wrong, but my speculation is that it may have something to do with the cleaning and you needing to foul the barrel up again. I know my barrels speed up after a break in period of about 60-100 rounds. Not sure if' that will account for that kind of difference, but it could be one factor...

 

[702lineman]

Looks like the DA between the two outings was quite different. I'd have to calculate it but you were 600+ ft less altitude and about 40+ degrees different. DA was way lower on the last one. It will make a pretty big difference on a 308

 

[702lineman]

Yup, looks like your da was about 4400 on the first trip and about 600 something on second. That's a huge swing. For sure would be your problem.

 

[richthe1]

Thanks for the feedback so far, guys. Hard to diagnose from afar, but DA seems to be one thing I can try to address before I head to the range again next weekend. I’m using BallisticsARC for my ballistic calculator. Would it be a good idea for me to try to find a solution in BallisticsARC that allows me to match the DOPE for first trip at 4400 DA and then by simply changing the DA to 600 it would reflect the DOPE from the second trip? Or would that be trying to do too much with too little?

Thanks for the feedback. This long range shooting stuff is harder than I thought!

 

[Deleted member 113831]

Your environmentals would only have accounted for 0.4 - 0.5 mils. You had something else going on as well.

 

[702lineman]

His zero was off .1 at 100 so there is a little bit and yeah @Skookum right, I doubt that is all of it.

 

[richthe1]

I’ll be honest, I’m pretty clueless as to what the other part(s) could have been.

-As mentioned above it could have been the newly cleaned barrel and needing more rounds down the tube to get it back to it’s normal velocity. When I go out next I will hook my magnetospeed up and see if velocity is a part of it.

-As mentioned above it could have been optical effects. My first outing location it was very bright. We were shooting toward the south and the sun was out most of the day. At the second outing location it was overcast almost all day and I started early in the morning. I’ve heard optical effects can effect where the target appears in your optic but don’t know much about it.

 

[bax]

Here are some general comments about what you observed. This is all about bullet drop caused by air drag. As air density increases, so does drag and drop.

Colder air is more dense than warm air - trip 1 was 97F, trip 2 was about 50F, that is a pretty large swing. For trip 1 the air was less dense so imposed less drag on the bullets - less drag equals less drop at each distance. Trip 2 had more-dense air - more drag equals more drop at each distance.

Air density is only slightly affected by humidity and the values were pretty much the same so similar effect. FWIW humid air is less dense than dry air.

Barometric pressure is a measurement of air density - the uncertainty shown for trip 1 might be a little or a lot. As pressure increases, density and drag and drop increase. Keep in mind, barometric pressure has two values. Station pressure is the actual measurement in inches of mercury or hPa. Sea-level pressure is station pressure adjusted for altitude, this gets tricky if the measurement device doesn't know its precise altitude. If you can get it, station pressure is best, no fiddling around with standard atmosphere and lapse rates and guessing elevation.

The International Civil Aviation Organization ICAO has a "standard atmosphere" it is 29.92 inches of mercury at 59F (or 15C and 1013.25 hPa) with no water vapor. Part of the adjustment from station pressure to sea-level uses a factor called "lapse rate" or the rate that the temperature drops as you increase in altitude - 1.8C per 1,000 feet or about 3.2 F per 1000. Air pressure drops about 1 inch of mercury per 1,000 foot elevation gain. You can see how tricky this can get. Your 500 altitude change is worth half an inch of mercury pressure increase and about 0.9 C temperature drop from trip 1 to trip 2.

I'm with skookum, I don't see the atmospherics accounting for a full mil at 800. On my rifle, when fired at the same elevation (959 feet), 88F to 40F is worth 0.3 mils when shooting steel at 800. That is measured gun data: .308 175 SMK, MV=2615 fps. That is pretty close to FGMM so WTH?