Need help. Impacting low.

[stick slinger]

I couldn't find quite the info I was looking for when I searched.

Question:
How much effect can mirage have to cause low impacts?

Info to save questions.
Gun: horizon carbon venatic 7prc, with at least 300 rounds through it
Scope: maven RS5 SFP ( I know, i know)

Conditions: I got out later than usual because I chose to run my 4.6 mile loop first, and deal with shooting in the heat.
0930 MDT
90 degrees, hot and sunny, intense sun.
15% humidity (based on weather channel)
Wind 2-5mph 9oclock ( no instrument)
Gun is clean and fouled with a few shot.

Scenario.
This is my primary shooting spot. I have thousands of acres of BLM right behind my house. I have shot this 662yd target many times and typically my primary ammo, prints a sub MOA horizontal group holding 10.5MOA elevation. At 100yd this ammo is always less than 0.5in.

I've been shooting my practice/back up hunting rounds the last few months because my primary rounds are expensive (.284 169gr HH) and propelled by RL25 (can't buy).

Typically at this 662 Target, my hold is 10.5MOA, and it is spot on. When I backed out to 902yd last week shooting this practice ammo my impacts were all high, when reviewing the footage through my spotting scope. My chart called for 18.3MOA, and when I dialed back to 17, I started making consecutive hits. I wasn't sure what to think of this. Then I shot at 500, a few days later and was also high 0.5 MOA. But I knew my gun needed cleaned, so I cleaned it last night.

Today, I tested some of my primary ammo (because I just reloaded a batch, for the first time in a while, and before I load it up a big pile of it, I wanted to test it.) the conditions were good, as detailed above.l, so I went ahead and shot distance first. I estimated 1MOA of wind dialed 10.5MOA and shot 3 rounds that felt good. Then, as the wind started to pick up, I set up my 100 yard Target, zered out my scope, and shot 0.5MOA group 1.0 in above the bullseye (200 yard zero). So my zero is good both with rifle, ammo, and scope.

When I checked my paper at the 662 Target, I found all of my impacts were 4in group directly below the bullseye 8in (1.1MOA), low.

Problem:
So how much effect can mirage have? I typically don't shoot when it's nearly this warm / sunny. Or am I having a tracking issue? What's the best way to sort this out? Shoot the square? I'll go take a cold bore shot before it gets too warm tomorrow.

Any insight here is appreciated!

 

[eric1115]

I would say a good starting point is to shoot more than 3 rounds per group.

They will lie to you. Zeroing or trying to true data with small sample sizes ends up burning more powder. 10 round zero group and then a 10 round group at 600-800 will give you more usable information than 10 different 3 round groups at 10 different distances.

I would start there, and once you have solid data to analyze then start working on figuring out where the discrepancy between expected and as-shot corrections might be coming from.

 

[Nine Banger]

The gun I'm shooting now tends to throw high and to the left once barrel shimmer is cranking.

The natural thermal mirage is going to manifest as moving the same direction as the wind, at least here where it's really humid.

At 662 yards 4" is only .1 mil...so is that about .5 MOA? Can you outshoot that?

My guns are grouping 8-9" if I shot 30 rounds at 662 yards.

I think all you can do is a 10-30 round zero check and then make sure your muzzle velocity is right in your calculator.

 

[WeiserBucks]

holding 10.5MOA elevation.

Are you actually holding or dialing?

 

[Wrench]

Chasing the blob is a skill. The 9 o'clock wind is playing into this as well....did you count in AJ?

 

[Wrench]

.....and what's your station pressure difference?

 

[stick slinger]

I would say a good starting point is to shoot more than 3 rounds per group.

They will lie to you. Zeroing or trying to true data with small sample sizes ends up burning more powder. 10 round zero group and then a 10 round group at 600-800 will give you more usable information than 10 different 3 round groups at 10 different distances.

I would start there, and once you have solid data to analyze then start working on figuring out where the discrepancy between expected and as-shot corrections might be coming from.

Thank you for the reply. I will try more shots per group.

I have shot lots at these targets, but with this ammo it's been awhile. I figured since I was still zeroed by 3 round group at 100 then my 662 hold would hit true too as it has many times in the past. I had the chrono too, since I hadn't loaded these hunting loads in awhile and they were only 17fps fast in the heat today from all my documented speeds from previous batches. ES 25, SD 10. So the ammo seemed quality too, but again only 6 shot string

 

[stick slinger]

The gun I'm shooting now tends to throw high and to the left once barrel shimmer is cranking.

The natural thermal mirage is going to manifest as moving the same direction as the wind, at least here where it's really humid.

At 662 yards 4" is only .1 mil...so is that about .5 MOA? Can you outshoot that?

My guns are grouping 8-9" if I shot 30 rounds at 662 yards.

I think all you can do is a 10-30 round zero check and then make sure your muzzle velocity is right in your calculator.

I'm Happy with the group, but it was 8in low. It's usually just a little horizontal group and rarely more than 2in high or low.

No I can't shoot better than that. Just confused that it's suddenly low.

Thanks for your help

 

[stick slinger]

.....and what's your station pressure difference?

Wrench,

I don't know that either AJ or station pressure is.

I don't usually get too technical with this. I've had great luck just making high quality ammo. Shooting in reasonable conditions. And consistently ringing steel 662-902, and one shot kills 410 and under. I've never taken a longer shot at an animal than 410. I don't have a kestrel or ballistics rangefinder.

I just watch the dust from my ATV as I drive from the target to shooting location. Make a wind call, look at my chart and I guess, usually get lucky.

That process today yielded a group 8in low when it's usually spot on at this 662 Target. As Eric1115 pointed out, I guess it's a pretty small sample size today. Groups at 100 and 662 were excellent, just not where I wanted to hit at 662. I usually don't deal with that much mirage, and have no idea how I can effect POI.

All help appreciated! Thanks

 

[stick slinger]

Are you actually holding or dialing?

Dial 10.5
Hold 1moa wind

 

[dårlig jeger]

Have you done a tracking test on a tall target at 100 to verify rtz and elevation?

 

[stick slinger]

Have you done a tracking test on a tall target at 100 to verify rtz and elevation?

Not yet. Scope seemed to track fine the last 300 rd. But it was a little wonky the last 2 outings, and then again today. It apparently tracks back to zero fine, based on a 3 shot group. (I'd never adjusted the scope to shoot my practice ammo as the POI is so close that I didn't bother) . Then today with this small batch of my hunting ammo, my zero was still spot on, based on 3 shot group. And small group at 662, wind appropriate, just 8in lower than usual....

Now I'm starting to wonder if I didn't dial a full 10.5moa? Haha. I'm sure I did, but I've been wrong before!

 

[JohnJohnson]

I don't know that either AJ or station pressure is.

AJ means aerodynamic jump. If the wind is coming from 3 o'clock it pushes your bullet down a tiny bit. If it's coming from 9 o'clock, it pushes it up a tiny bit. So for instance with my 6.5CM if I'm dealing with a 10 mph crosswind coming from 3 o'clock it pushes my bullet down 0.08 MRAD at 100 yards. That's like 0.3" which is not nothing once you start going to longer ranges. So aerodynamic jump can even have an effect on your zero as well if you're zeroing with crosswinds and don't know it.