Does rain reduce computed drop?

[bax]

Shooting last Sunday. Conditions: rain visible in scope - moderate rate; 49 F ; 68 % RH; 28.24 inHg absolute. Bullet: Hornady, .308 230 gr A-Tip, 2700 FPS, barrel 1:9, scope height 2 inches. The rain was not falling straight down, I needed about 0.4 right most of the time - range 0.2 to 0.8 right, it was pretty easy to read the rain and estimate the windage.

Computed trajectory at 1k is 7.7 mils. Actual trajectory 7.1 mils, I shot 25 rounds - 7.1 is a consistent number. Using Applied Ballistics on Android. Shooting was from a 60 F building (yeah, I know I'm a slacker). No rain drops on muzzle. I can't explain where the 0.6 mils came from. I know about rain drops on the muzzle causing a bump, that is not the case here.

Any ideas?

 

[_Windrider_]

Who cares what it computed, make the computer match the actual. Tweak MV to line up around 600 and at 1000 maybe tweak BC to refine it until you get 7.1 at 1k.

We make the calculator match actual, not the other way around.

 

[moaorbust]

I assume your data is trued on a normal sunny day. My first thought would be a tail wind at some point. How well were you able to see every hundred yards or so? Any berms? At my local range the wind can be left to right but the berns cause a tunnel effect you have to know to account for.

 

[bax]

Who cares what it computed, make the computer match the actual. Tweak MV to line up around 600 and at 1000 maybe tweak BC to refine it until you get 7.1 at 1k.

We make the calculator match actual, not the other way around.

In non-rainy weather, AB is right on. I used a Kestrel to determine temp, RH, and pressure.

I used this technique to determine MV: shoot every 100 yds out to 1k, write down the scope adjustment for each distance. AB knows about the bullet and the barrel. Pick a MV and compute trajectory. Change the AB muzzle velocity until the computed trajectory matches reality. In most weather, AB is right on. But not on Sunday in the rain.

 

[bax]

I assume your data is trued on a normal sunny day. My first thought would be a tail wind at some point. How well were you able to see every hundred yards or so? Any berms? At my local range the wind can be left to right but the berns cause a tunnel effect you have to know to account for.

Yeah ... no. According to AB, 100 mph will give me 0.2. Enough tailwind to cause six tenths would blow the building down, I'm pretty sure I would have noticed.

The range is Youngs in northern Indiana. From the firing line out to about 450 there are berms and trees and the wind does blow up and down the range. Between 500 and 900, the wind is usually either 9 to 10 o'clock or 3 to 4 o'clock. I was watching the wind blow the rain through the scope and I was getting horizontal when I didn't pay close attention. There was no noticeable wind noise.

I'm usually happy with AB calculations this just seemed odd. The result is in my book.

 

[_Windrider_]

In non-rainy weather, AB is right on. I used a Kestrel to determine temp, RH, and pressure.

I used this technique to determine MV: shoot every 100 yds out to 1k, write down the scope adjustment for each distance. AB knows about the bullet and the barrel. Pick a MV and compute trajectory. Change the AB muzzle velocity until the computed trajectory matches reality. In most weather, AB is right on. But not on Sunday in the rain.

I would conclude that it was you or something in the system then. Rain shouldn’t affect trajectory or wind corrections to that extreme. It would effect pressure in the chamber if your ammo got wet or the chamber was wet. The bullet isn’t even getting wet when going through the rain due to the pressure “bubble” that builds in supersonic flight, in transonic and subsonic there could be an issue which perhaps is your problem since a 308 at 1k is probably in transonic range.

 

[bax]

300 PRC, at 1k I am still running 1,725 FPS. I'm thinking a 230 grain bullet fired from a 308 running at 2700 FPS would be pretty miraculous. There is no room for the powder

I agree, it could have been me but ... I fired 25 rounds. Because I started at +7.7 and I was by myself to there was no-one to tell me where I was impacting, it took a few to get on steel - 3 or 4, I think (the backstop is null, you can't see splash on it and the rain washed out the visual). I shot some on a 36-inch plate, some on a 24-inch plate, and some on a 12-inch plate. I think I fired 10 or 12 rounds on the 12. I had 6 hits on the 12. A 12-inch plate is three tenths in diameter. Six tenths is about 21.6 inches. It would be the most astounding luck if it was me and I still got that many hits on 12. And I was shooting a three-tenths group on the 36-inch plate. So, it could have been me but - like some of our recent elections - why was the error all in one direction?

The other thing it could have been - I took no zero-check shots. Maybe I bumped something and the gun was off by six tenths. I plan to check that next time.

I like the bumped scope, it explains everything. Problem is, I don't think I did it I will check.

 

[DangerRanger]

No way this could possibly be a “lights up sights up”situation is there?

Not a new barrel is it?

 

[bax]

No way this could possibly be a “lights up sights up”situation is there?

Not a new barrel is it?

re: lights - nope. This is near southern Michigan. Winter brings gray skies - rain is not that different. I shoot in it all the time.

Barrel is Bartlein, about 300 rounds. For me, Bartlein barrels are The Best.

I read on here that sometimes when a barrel breaks in, it gets a little extra velocity. According to AB, if the velocity went from 2700 to about 2775, then +7.1 or +7.2 would be the result. I will keep track.